RawTherapee 5.4 AppImage font issue

@Carmelo_DrRaw a987c280aea2bbbf18c0917e550cc4c4 RawTherapee-dev-5.4-266-ga84ce624.AppImage ran without crashing, but it was very slow in some ways. Panning the main preview or adjusting a curve felt close to normal, but right-clicking on a different thumb in the File Browser takes 2 seconds to show the context menu, and switching from the Editor tab to the File Browser tab, and vice-versa, takes 10 seconds and makes my whole system jerky. No warnings nor errors appear in the console while this happens. Preferences also takes about 10 seconds to appear and over a second to close.

$ ./RawTherapee-dev-5.4-266-ga84ce624.AppImage        
System stdc++ library: "/usr/lib/gcc/x86_64-pc-linux-gnu/6.4.0/libstdc++.so.6"
System stdc++ library version: "3.4.22"
Bundled stdc++ library version: "3.4.22"
Newest stdc++ library version: "3.4.22"
Using system stdc++ library
fcv: "1.10.1"  fcv2: "1.9.2"
echo "1.10.1 1.9.2" | tr " " "\n" | sort -V | tail -n 1
Newest fontconfig library version: "1.10.1"
Using system fontconfig library
        linux-vdso.so.1 (0x00007ffd56f91000)
        libcanberra-gtk3.so.0 => /tmp/.mount_ZfVs38/usr/lib/libcanberra-gtk3.so.0 (0x00007fdd45a63000)
        libX11.so.6 => /usr/lib/libX11.so.6 (0x00007fdd4570c000)
        libcanberra.so.0 => /tmp/.mount_ZfVs38/usr/lib/libcanberra.so.0 (0x00007fdd454fc000)
        libgtk-3.so.0 => /tmp/.mount_ZfVs38/usr/lib/libgtk-3.so.0 (0x00007fdd44bf0000)
        libgdk-3.so.0 => /tmp/.mount_ZfVs38/usr/lib/libgdk-3.so.0 (0x00007fdd4492f000)
        libatk-1.0.so.0 => /tmp/.mount_ZfVs38/usr/lib/libatk-1.0.so.0 (0x00007fdd44709000)
        libgio-2.0.so.0 => /tmp/.mount_ZfVs38/usr/lib/libgio-2.0.so.0 (0x00007fdd44361000)
        libpangocairo-1.0.so.0 => /tmp/.mount_ZfVs38/usr/lib/libpangocairo-1.0.so.0 (0x00007fdd44154000)
        libgdk_pixbuf-2.0.so.0 => /tmp/.mount_ZfVs38/usr/lib/libgdk_pixbuf-2.0.so.0 (0x00007fdd43f31000)
        libcairo-gobject.so.2 => /tmp/.mount_ZfVs38/usr/lib/libcairo-gobject.so.2 (0x00007fdd43d29000)
        libpango-1.0.so.0 => /tmp/.mount_ZfVs38/usr/lib/libpango-1.0.so.0 (0x00007fdd43adc000)
        libcairo.so.2 => /tmp/.mount_ZfVs38/usr/lib/libcairo.so.2 (0x00007fdd437ac000)
        libgobject-2.0.so.0 => /tmp/.mount_ZfVs38/usr/lib/libgobject-2.0.so.0 (0x00007fdd43558000)
        libglib-2.0.so.0 => /tmp/.mount_ZfVs38/usr/lib/libglib-2.0.so.0 (0x00007fdd43244000)
        libexpat.so.1 => /usr/lib/libexpat.so.1 (0x00007fdd43015000)
        libfftw3f.so.3 => /tmp/.mount_ZfVs38/usr/lib/libfftw3f.so.3 (0x00007fdd42c10000)
        libfftw3f_omp.so.3 => /tmp/.mount_ZfVs38/usr/lib/libfftw3f_omp.so.3 (0x00007fdd42a0a000)
        libgiomm-2.4.so.1 => /tmp/.mount_ZfVs38/usr/lib/libgiomm-2.4.so.1 (0x00007fdd4265d000)
        libglibmm-2.4.so.1 => /tmp/.mount_ZfVs38/usr/lib/libglibmm-2.4.so.1 (0x00007fdd423e1000)
        libsigc-2.0.so.0 => /tmp/.mount_ZfVs38/usr/lib/libsigc-2.0.so.0 (0x00007fdd421db000)
        libgthread-2.0.so.0 => /tmp/.mount_ZfVs38/usr/lib/libgthread-2.0.so.0 (0x00007fdd41fda000)
        libgtkmm-3.0.so.1 => /tmp/.mount_ZfVs38/usr/lib/libgtkmm-3.0.so.1 (0x00007fdd41868000)
        libatkmm-1.6.so.1 => /tmp/.mount_ZfVs38/usr/lib/libatkmm-1.6.so.1 (0x00007fdd4161b000)
        libgdkmm-3.0.so.1 => /tmp/.mount_ZfVs38/usr/lib/libgdkmm-3.0.so.1 (0x00007fdd413c9000)
        libpangomm-1.4.so.1 => /tmp/.mount_ZfVs38/usr/lib/libpangomm-1.4.so.1 (0x00007fdd4119c000)
        libcairomm-1.0.so.1 => /tmp/.mount_ZfVs38/usr/lib/libcairomm-1.0.so.1 (0x00007fdd40f75000)
        libiptcdata.so.0 => /tmp/.mount_ZfVs38/usr/lib/libiptcdata.so.0 (0x00007fdd40d69000)
        libjpeg.so.9 => /tmp/.mount_ZfVs38/usr/lib/libjpeg.so.9 (0x00007fdd40b2f000)
        liblcms2.so.2 => /tmp/.mount_ZfVs38/usr/lib/liblcms2.so.2 (0x00007fdd408d6000)
        libpng12.so.0 => /tmp/.mount_ZfVs38/usr/lib/libpng12.so.0 (0x00007fdd406b0000)
        libz.so.1 => /lib64/libz.so.1 (0x00007fdd40497000)
        libtiff.so.5 => /tmp/.mount_ZfVs38/usr/lib/libtiff.so.5 (0x00007fdd4022f000)
        liblensfun.so.1 => /tmp/.mount_ZfVs38/usr/lib/liblensfun.so.1 (0x00007fdd40010000)
        libstdc++.so.6 => /usr/lib/gcc/x86_64-pc-linux-gnu/6.4.0/libstdc++.so.6 (0x00007fdd3fbe0000)
        libm.so.6 => /usr/lib/libm.so.6 (0x00007fdd3f8c7000)
        libgomp.so.1 => /usr/lib/gcc/x86_64-pc-linux-gnu/6.4.0/libgomp.so.1 (0x00007fdd3f692000)
        libgcc_s.so.1 => /usr/lib/gcc/x86_64-pc-linux-gnu/6.4.0/libgcc_s.so.1 (0x00007fdd3f47b000)
        libpthread.so.0 => /lib64/libpthread.so.0 (0x00007fdd3f25a000)
        libc.so.6 => /lib64/libc.so.6 (0x00007fdd3ee98000)
        libvorbisfile.so.3 => ././/lib/libvorbisfile.so.3 (0x00007fdd3ec90000)
        libvorbis.so.0 => ././/lib/libvorbis.so.0 (0x00007fdd3ea63000)
        libogg.so.0 => ././/lib/libogg.so.0 (0x00007fdd3e85a000)
        libltdl.so.7 => ././/lib/libltdl.so.7 (0x00007fdd3e650000)
        libdl.so.2 => /lib64/libdl.so.2 (0x00007fdd3e44c000)
        libxcb.so.1 => /usr/lib/libxcb.so.1 (0x00007fdd3e21b000)
        libgmodule-2.0.so.0 => ././/lib/libgmodule-2.0.so.0 (0x00007fdd3e018000)
        libXi.so.6 => ././/lib/libXi.so.6 (0x00007fdd3de08000)
        libXdamage.so.1 => ././/lib/libXdamage.so.1 (0x00007fdd3dc05000)
        libXfixes.so.3 => ././/lib/libXfixes.so.3 (0x00007fdd3d9ff000)
        libatk-bridge-2.0.so.0 => ././/lib/libatk-bridge-2.0.so.0 (0x00007fdd3d7d1000)
        libepoxy.so.0 => ././/lib/libepoxy.so.0 (0x00007fdd3d4dc000)
        libpangoft2-1.0.so.0 => ././/lib/libpangoft2-1.0.so.0 (0x00007fdd3d2c7000)
        libfontconfig.so.1 => /usr/lib/libfontconfig.so.1 (0x00007fdd3d07b000)
        libfreetype.so.6 => ././/lib/libfreetype.so.6 (0x00007fdd3cdda000)
        libXext.so.6 => ././/lib/libXext.so.6 (0x00007fdd3cbc8000)
        librt.so.1 => /lib64/librt.so.1 (0x00007fdd3c9bf000)
        libresolv.so.2 => /lib64/libresolv.so.2 (0x00007fdd3c7a8000)
        libpixman-1.so.0 => ././/lib/libpixman-1.so.0 (0x00007fdd3c500000)
        libEGL.so.1 => /usr/lib/libEGL.so.1 (0x00007fdd3c2c1000)
        libxcb-shm.so.0 => ././/lib/libxcb-shm.so.0 (0x00007fdd3c0be000)
        libxcb-render.so.0 => ././/lib/libxcb-render.so.0 (0x00007fdd3beb5000)
        libXrender.so.1 => ././/lib/libXrender.so.1 (0x00007fdd3bcab000)
        libGL.so.1 => /usr/lib/libGL.so.1 (0x00007fdd3ba2d000)
        libffi.so.6 => ././/lib/libffi.so.6 (0x00007fdd3b825000)
        libpcre.so.3 => /tmp/.mount_ZfVs38/usr/lib/libpcre.so.3 (0x00007fdd3b5e7000)
        liblzma.so.5 => /tmp/.mount_ZfVs38/usr/lib/liblzma.so.5 (0x00007fdd3b3c5000)
        libjbig.so.0 => /tmp/.mount_ZfVs38/usr/lib/libjbig.so.0 (0x00007fdd3b1b7000)
        /lib64/ld-linux-x86-64.so.2 (0x00007fdd45c67000)
        libXau.so.6 => /tmp/.mount_ZfVs38/usr/lib/libXau.so.6 (0x00007fdd3afb3000)
        libXdmcp.so.6 => /tmp/.mount_ZfVs38/usr/lib/libXdmcp.so.6 (0x00007fdd3adad000)
        libatspi.so.0 => ././/lib/libatspi.so.0 (0x00007fdd3ab7d000)
        libdbus-1.so.3 => ././/lib/libdbus-1.so.3 (0x00007fdd3a938000)
        libharfbuzz.so.0 => ././/lib/libharfbuzz.so.0 (0x00007fdd3a697000)
        libxcb-dri2.so.0 => ././/lib/libxcb-dri2.so.0 (0x00007fdd3a492000)
        libX11-xcb.so.1 => ././/lib/libX11-xcb.so.1 (0x00007fdd3a290000)
        libxcb-dri3.so.0 => ././/lib/libxcb-dri3.so.0 (0x00007fdd3a08d000)
        libxcb-xfixes.so.0 => ././/lib/libxcb-xfixes.so.0 (0x00007fdd39e86000)
        libxcb-present.so.0 => ././/lib/libxcb-present.so.0 (0x00007fdd39c83000)
        libxcb-sync.so.1 => ././/lib/libxcb-sync.so.1 (0x00007fdd39a7d000)
        libxshmfence.so.1 => ././/lib/libxshmfence.so.1 (0x00007fdd3987b000)
        libwayland-client.so.0 => /usr/lib/libwayland-client.so.0 (0x00007fdd3966b000)
        libgbm.so.1 => /usr/lib/libgbm.so.1 (0x00007fdd3945b000)
        libwayland-server.so.0 => /usr/lib/libwayland-server.so.0 (0x00007fdd39245000)
        libdrm.so.2 => /usr/lib/libdrm.so.2 (0x00007fdd39031000)
        libglapi.so.0 => ././/lib/libglapi.so.0 (0x00007fdd38e0a000)
        libxcb-glx.so.0 => ././/lib/libxcb-glx.so.0 (0x00007fdd38bf3000)
        libXxf86vm.so.1 => ././/lib/libXxf86vm.so.1 (0x00007fdd389ed000)
        libSM.so.6 => /usr/lib/libSM.so.6 (0x00007fdd387e4000)
        libICE.so.6 => /usr/lib/libICE.so.6 (0x00007fdd385c6000)
        libuuid.so.1 => /lib64/libuuid.so.1 (0x00007fdd383c1000)
        libbsd.so.0 => /usr/lib/libbsd.so.0 (0x00007fdd381aa000)
Gtk-Message: 18:31:26.065: Failed to load module "canberra-gtk-module"

(rawtherapee:27830): Gtk-WARNING **: 18:31:26.171: Theme parsing error: gtk.css:68:35: The style property GtkButton:child-displacement-x is deprecated and shouldn't be used anymore. It will be removed in a future version

(rawtherapee:27830): Gtk-WARNING **: 18:31:26.171: Theme parsing error: gtk.css:69:35: The style property GtkButton:child-displacement-y is deprecated and shouldn't be used anymore. It will be removed in a future version

(rawtherapee:27830): Gtk-WARNING **: 18:31:26.171: Theme parsing error: gtk.css:70:34: The style property GtkCheckButton:indicator-size is deprecated and shouldn't be used anymore. It will be removed in a future version

(rawtherapee:27830): Gtk-WARNING **: 18:31:26.171: Theme parsing error: gtk.css:71:36: The style property GtkCheckMenuItem:indicator-size is deprecated and shouldn't be used anymore. It will be removed in a future version

(rawtherapee:27830): Gtk-WARNING **: 18:31:26.172: Theme parsing error: gtk.css:73:46: The style property GtkScrolledWindow:scrollbars-within-bevel is deprecated and shouldn't be used anymore. It will be removed in a future version

(rawtherapee:27830): Gtk-WARNING **: 18:31:26.172: Theme parsing error: gtk.css:76:30: The style property GtkExpander:expander-size is deprecated and shouldn't be used anymore. It will be removed in a future version

(rawtherapee:27830): Gtk-WARNING **: 18:31:26.181: Theme parsing error: gtk.css:83:29: The style property GtkStatusbar:shadow-type is deprecated and shouldn't be used anymore. It will be removed in a future version