Ansel’s UI is definitely faster. A particular metric which you can find in his (very rude) performance blog posts is when switching from lighttable to darktable, this is pretty slow in darktable, at least one second, independent of hardware, and nearly instant in Ansel. I am not saying Ansel is better, just that in this particular example, and some other UI bits, a lot of optimization has been done to ease out a few delays which are pretty uncomfortable in dt. Obviously the pipeline is likely faster in darktable after all the optimizations.
From his post, although it is not clear if he is comparing with the latest version or 4.0 which Ansel is compatible with.
The pixel pipeline has been completely overhauled: modules run about 1.8× faster on CPU and 1.6× faster on GPU than darktable, and — more importantly — the pipeline cache was rewritten so that disabling a module, using a color picker or re-exporting an already-exported image does not recompute the whole thing. That makes Ansel 4 to 100× faster than darktable to refresh the image. Memory management was rewritten too: Ansel knows exactly how much RAM it uses and stays within the budget you set, instead of darktable’s hand-wavy “performance profiles” that have no predictability.
The whole lighttable, thumbnail and mipmap-cache stack was rewritten. Concretely: starting the app and drawing a full collection is roughly 3.5× faster, switching between lighttable and darkroom is near-instant, and scrolling a large collection no longer stutters. The right sidebar was removed (its content moved to the global menu and the left panel), culling became the Restrict to selection filter, the filtering toolbar was rebuilt as plain inclusive toggle buttons, the zoomable lighttable came back (Columns + Zoom with barycentric auto-pan), and selection became explicit and What You See Is What You Get — no more metadata loss from a dangling mouse cursor.