I timed RT on my Win10-machine:
After re-boot, double-click on RT icon on desktop.
RawTherapee 4.2.1234 (GTK3-version) starts in 9 seconds.
I have not set any compatibility modes.
Afterthoughts:
Do all other programs start slowly or is it just RT?
Are you running the SSDs in IDE or AHCI mode?
Some more timings for comparison:
On my rebooted 4 years old Win7 desktop (i5, 16 GB, 2 x HD) 4.2.1069 (master) starts in 8 seconds, after closing and reopening, in 3 seconds. On my Win10 modest laptop (i3, 8 GB, SSD + HD, from grocery store), this takes 6 s and then 4 s.
Gtk 3 version was slower, when I tried it for the last time, but not much, about a second or so.
Edit: was slower to start
As @Claes asked, have you tried other Gtk±using programs, such as GIMP? How fast they they start?
FYI: The Gtk3 version takes slightly longer to start (1-3 seconds, depending on your system), but apart from startup time it runs just as fast as the Gtk2 version.
Just in case:
I don’t remember Windows 10 being available for upgrade from Vista. Was it?
Also, you probably have some kind of anti-virus installed, plus the SmartScreen “feature”, that prevented me from installing multiple open source programs, before I disabled it. (It checks “unknown” programs against the MS whitelist.) Did you check this?
Thanks for testing gtk2 branch. I guess it’s related to this issue. If you have a filesystem with lots of folders this damn FileChooserButtons take a lot of time. Maybe that’s even worse on Win10
That was it!!! Earlier, I answered that I didn’t see any A: drive listed anywhere in Windows, but that was before I looked in the Bios.
Claes asked me to check whether the SSD’s run in IDE or AHCI mode. So, I opened the BIOS and indeed all disks run in AHCI mode. Doing so, I saw a question whether there is or not a floppy drive on A: that was set to YES. I selected NO and rebooted.
Now the gtk2 build starts in 6 s. I will try the gtk3 build next.
Thank’s so much every one for your help. This problem is solved as far as I am concerned.
Yes it was and I am fortunate it was because I don’t boot Windows that often on my main PC. I feel Windows10 is much better that Vista and more like Windows7 which is what I use on my slates.
Thank you for information, and I agree, Win 10 should run better than Vista on the same machine, if there are no compatibility issues.
Programs are normally lively on my setup except for RT. That was before I did a small change in a BIOS setting.
Thanks to your questions and ilias_giarimis suggestion, I found out that there is an option in the BIOS asking whether there is or not a Floppy connected. After setting that option properly (I have no floppy installed) RT now starts quickly.
(Edit) Now the gtk3 and the gtk2 build both start in less than 7 s. 
Nice, that is something to remember.
@rlx009 Fine 
For me the Drive A was listed in “My Computer” and dectivating it from Device Manager was enough.
Ingo had pointed me to the right direction Long time to load Rt on Win 10 64bit - #22 by heckflosse but now that link is dead.
It’s my fault that I didn’t reported the solution in time 
@Morgan_Hardwood although the right solution could be correcting GTK behavior by GTK gurus, until then, an item in RT_Wiki and in Rawtherapee.com/Downloads would help …
@ilias_giarimis agreed.
@rlx009 you haven’t answered whether you tried GIMP or other Gtk±using programs. Could you re-enable drive A and test?
Notable software which use Gtk:
I don’t know which of those use the GtkFileChooser, so it would be best if you tried several of them.
And another thing. If you edit your “options” file, and set verbose=true, then run RawTherapee from the command prompt, can you paste the log here and mark at which point it paused for a long time?
Sorry, I missed your question. I didn’t have any of those on my main PC since I would normally use them in Linux and maybe I didn’t always have much success in the past when I tried them on Windows – except for MyPaint that works beautifully on my tablet PC’s. Anyway, following your question I loaded Gimp, Indscape and Pidgin on my Win10 PC and here are the results.
By the way the exact wording for the Bios option in my case is “Legacy diskette A” and the options offered are “Disable”, …, “1.44M 3.5in”.
After setting “Legacy diskette A” to “1.44M 3.5in” in the BIOS,
After setting “Legacy diskette A” to “Disabled” in the BIOS,
I will have to repeat the test for Gimp with Diskette A enabled before I can conclude.
(Edit) Doing the test once again with Gimp.
After setting “Legacy diskette A” to “1.44M 3.5in” in the BIOS,
The conclusion is that out of those four GTk based programs, only RT is impacted by the “Legacy diskette A” selection in the BIOS.
Now changing PC from my Win10 PC to my tablet PC running Win7. I have RT 32-bit installed (4.1.64) on that PC, and that one doesn’t even show a window after close to an hour ( I started it on the slate when doing those tests on my Win10 PC). MyPaint starts quickly on that same PC. Lots of mysteries…
a) Good!
b) But of course I will never forgive you that you now can start RT faster than I can 
That’s almost as using Windows95 
You shouldn’t have lead me to look in the BIOS, I would be still at 4 min slower than you 
.
OK! Just upgraded to latest 1234 gtk3 build. RT now starts in 6 s on that old TabletPC 
@rlx009 thank you for the detailed response. One thing I don’t get:
It clearly affects GIMP too. The “looking for fonts” splash screen has no GtkFileChooser so it doesn’t count, but the main GIMP window didn’t show after 6 minutes.
When on googles “gtkfilechooser slow” one finds a coal mine of posts and bug reports complaining about that widget’s abysmal speed. Strange that Gtk+ doesn’t do anything about it, those posts pertain to both Gtk2 and 3 spanning several years.
The first time I started Gimp on Windows, I didn’t see the main Gimp window because it was mostly off-screen on the left. That’s what I believe. There are icons all over the left part of my screen and the portion of the Gimp window on the screen as I saw it on my second try was a thin vertical line which was quite easy to miss. I saw that line the second time I tried because, at that point, I felt mystified and looked more closely.
It happened to me before on my TabletPC that I started a program and the window opened off-screen. That was the first time I met that issue on Win10.
That’s right, my experience is that Gimp starts quickly whatever that “Legacy diskette A” setting is set to in the Bios. RT is sensitive to that setting though.
I believe you are talking about delays when opening files from the Gtk file browser. Isn’t that a different issue than the slow start problem? I ask since I am new to those problems. I did a quick search on Google and found some people experiencing delays on NFS and Samba. My own experience using Gimp on Windows earlier today is that I found it surprisingly responsive, and I didn’t experience any abnormal delay. I didn’t try opening a file on the network yet. I will do that the next time I boot my PC. Well I just tried minutes ago to open a network file from RT on the TabletPC but the network locations are not listed within RT.
We’d need to send you a build with debug messages displayed at various stages while loading to be sure. Speaking of which, have you had time to try the “verbose=true” test I mentioned?