Well, I tried your save game and ran into the same problems. Unfortunately, all I can tell is that this particular slowdown doesn't seem to be caused by graphics performance issues, but rather some resource loading bug in the game, since the "loading" clock icon keeps popping up repeatedly, as if it was trying to load the same resources over and over again.

As for the crash, I checked it briefly with a debugger and it also didn't occur inside any ddraw calls, so it's most likely also "just" a game bug.

I suppose either of these bugs could have been introduced by the unofficial patches, but since your save game refuses to load without them, I couldn't test that.

I even tried running the game in a Windows XP VirtualBox and still had the exact same issues.

The bug report thread for UAP does mention that there may be bugs and even hangups if you use a save game from a version of the game that didn't have the unofficial patch installed. Did you continue a saved game from before installing the UAP?

Sorry, I'm afraid I have no solution for this...

Oh no, that's the issue. I did install the patch while in the middle of that game.

Ok, I managed to find a workaround for the time being.

Ultimately this indeed appears to be a problem with a particular resource found in Arcanum\modeules\Arcanum.PATCH1.
The affected resource is: Arcanum\maps\Arcanum1-024-fixed\68853695431.sec

Fortunately the workaround is simply to extract an older version of this resource from Arcanum.dat into the modules directory. To save you the headache I already did this and attached it as a zip file. Please ignore the fact that it's uploaded as a .png image (forum restrictions...), just change the extension from .png to .zip and extract it as usual.

It contains an "Arcanum1-024-fixed" directory, copy this directly to modules\Arcanum\maps in your Arcanum install directory, so that you'll end up with the following file within the install directory:
modules\Arcanum\maps\Arcanum1-024-fixed\68853695431.sec

Be aware that doing this may have unforeseen consequences - possibly even corrupting your future save games, so do it at your own risk (and definitely back up your saves first). Unless the problem is indeed caused by continuing an old unpatched saved game with the patch, it may be better to bring this issue to the more experienced Arcanum modders (or even the UAP author, if available) and ask them to fix the resource. Hopefully they could also reproduce this problem using your save game.

But otherwise, this seems to get rid of both the horrible slowdown and the crashes in the area...

Oh awesome! There we go, now I can walk around without crashes!
Thank you very much.
If I have any other issues, I think I'll seek out the help of Arcanum modders as you said.
But thank you! I'm excited to really get back to it!