Ok, well, I can see why you didn't want to draw that manually...
And as I suspected, it's pretty pixelated so you probably have 10's of thousands of right angles to process. Unfortunately, MT isn't smart enough to properly slice and render the map and basically has to process vision for all the tokens when a light source changes and all that so the more VBL you have on the map the worse it gets.
Now, with that said, with a server started, moving a token 40ft, I was able to get it down to less than 1 second. The trick was I had to remove the VBL for 2 of the maps... So you have a few options.
1. Redraw all the VBL by hand (yuck! And no idea on what the performance gain will end up being)
2. Put each map on a different MT Map and use teleporters (or transfer manually), not the most graceful but depends on how the maps hook up, I saw ladders so it may make sense they can't see the next map?
3. Create a few macros and dynamically generate/remove the VBL as needed.
Personally I'd go with #3. I did a quick test and using auto-generated vbl on the token performed just fine (for 1 map). Which makes sense, it's only grabbing 1-3 "Areas" of vbl at a time. In my test, performance degrades when the # of objects approaches a 50+ tokens.
PS I noticed the one map was off centered in the layout panel, restoring it made the token VBL show properly. Looks like we got a bug in the "preview" window for token vbl not handling anchor points properly. It's only a preview window bug and doesn't affect the actual VBL used...
So using auto-generated VBL, I'd store those tokens (maps) on a different map, and use a macro to move said map token to/from storage. As the players approach the new map, bring it over (performance degrades a bit) and once on the new map remove the old. It's pretty quick as you are just moving tokens (although you could store generated VBL on a token and draw/erase vbl using that, i think the other approach is cleaner).