I can guarantee you that running DotMM in Foundry is quite doable, I’ve been doing so for several months now.
It’s probably not Foundry itself, but something else in your set-up. Have these problems been there since day 1? If not, did you recently install a new module or update modules / game systems? Try disabling all modules and experimenting with turning them back on incrementally to find a possible culprit.
There are also client side settings that have enormous impact on performance. The framerate is set by default to 60 fps, but in my experience 20 fps is plenty - and much less intensive for laptops to run. Your players need to change this setting themselves while logged in, because it’s client specific.
Optimizing media is a good idea too, but I wonder if your players have computers that meet the minimum requirements...
If they do, you might want to check if the device that you host Foundry from meets the minimum requirements. Another tip is to ask your players to try using a different browser (preferably Chrome).
It may also be network related. Every time a token is moved, changes are written to disk. Does the device you host the game from have access to decent upload/download bandwidths? And is there enough free space left on the disk? Have you experimented with making a secondary world and asking others to join you there?
If the issues persist even then, then you’d might want to head over to Foundry’s Official Discord server to ask about your problems there.
Try Soundtale it has a lot of good stuff and its really easy to use.
For what you get for the small price tag its pretty good.
Alos you can get some great ambient sounds from a tone of free sound file websites.
Just type free "what your looking for" sound effect into google and you should be able to find something.
I run this module in Foundry VTT myself and don't find it too daunting to prep (outside of walls, oh god the walls), I used to use Roll20 but with the size of these maps and being a sucker for detailed maps have to use Foundry for higher resolution maps since the floors are gigantic.
Outside of initiative I let them travel down corridors and the entirety of the dungeon, they don't move themselves to fast (usually their max pace or dashing) as this gives them more engagement and doesn't make too far from the other players if they engage something (otherwise if they were following just make them in the marching order approach nearby as to not make them far out of combat).
There's alot of doors in the dungeon so having them locked or toggleable helps a bunch, most of the time the party will stop and check the door but that's also on your players (some of the doors in the dungeon being opened up directly can lead into immediate confrontation so the players might take it slow on approaching doors). The places that don't have doors, you can either manually toggle pause or use "Hey, Wait!" tiles to stop them from progressing which also toggles the game's pause function.
I personally only set triggers on traps and manually pause if they encounter something in direct line of sight that have spotted them through passive perception or something the book says would happen if they approach something, otherwise it's upto the players while I describe the scene.
I use the Hey Wait! module for traps and the like if they trigger them and its worked out alright so far.
Did you built the level yourself from the hardcopy or convert the module from roll20 etc?
Also did you review the Foundry optimization recommendations? https://foundryvtt.com/article/media/
It may be a wall and or lighting issue?
when I was able to play in person and wanted to have big maps I used this free website/program:
basically it takes a large image and splits it up into chunks that you can print out on normal sized paper, its great for making art which is what I think is intended, but you can also use it for dnd
I print them out using 'black only/grayscale" and they print less fuzzy. They print well.
Then what I do is draw as they go. I use a light tracing pad like this:
https://smile.amazon.com/gp/product/B07F147VF6/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
and draw the map as they explore. Using a battle mat with terrain tiles to make a room once a battle begins with minis.
You don't need to put the TV flat on the table. That setup can be really awesome, but until you get that properly going, just hook your laptop into a TV that everyone can see. Move a token around as they explore and everyone will see the map on the TV. When there's combat have someone quick draw out the room on a dryerase battle grid. Use actual minis on the battle grid on the table for combat. Then go back to just using the TV as a visual when out of combat.
I've used this method with dungeons of all sizes for several years. Works far better than printing out map portions, having PCs map things out, etc.
If you're not crafty and cannot afford a big TV and junk may I recommend getting the map pack off Amazon? https://www.amazon.com/Dungeons-Dragons-Waterdeep-Miscellany-Roleplaying/dp/0786966653/ref=sr_1_2?crid=WU5XK6P43G8F&keywords=dungeon+of+the+mad+mage&qid=1579235461&s=books&sprefix=dungeon+of+t%2Cprime-instant-video%2C163&sr=1-2
I personally use this by taking construction paper and covering the parts of a floor they haven't explored yet. Then I scan it with my printer and print off a player copy. Other than that I focus more on describing the rooms to them and only draw them out on our map grid (I think it's Chessex) when combat or something else dramatic happens.
There aren't any official laminated player maps that I know of. I used the ones from dndbeyond and printed and laminated them at home with these self-adhesive sheets.
If you do want to change a room's location later, Halaster could have just rearranged the dungeon on a whim. He could just be shifting things around to mess with players. So if the map they have represents a map they made, there's no reason it has to be the same when they return.
What program are you using to edit the maps?
My group and I have been using the Magic Item Purchasing rules from Xanathar's Guide to Everything
and Stronghold's and Followers from MCDM (Matt Colville)
In addition, I've given them extra incentive towards spending extracted gold by giving them XP for it.
So far the group has paid for 2 uncommon magic items. Along with purchasing the deed for an abandoned dwarven manor house with a defunct tower. They have also completely paid off the construction costs for a Wizards Tower and an Establishment from Strongholds, and they've put down 2000 gp into the construction of a Keep (5500 gp left on that).
We are still waiting on the construction of the Tower and The Establishment to be completed, 23 in-game days left on the Tower and 30 in-game days left on The Establishment. They just started construction on the Keep and have 109 in-game days left on it.
It occurs to me that these 13 double-sided cardstock sheets are not going to be "Dungeon Tiles" in the historic sense, but rather an entire dungeon layer will fit on each 8.5"x11" side. I guess you, as DM, could use them with dry erase markers to track the party's progress through each layer of the dungeon.
I can't think of how these tiles could be shown to or used by the party, then, as they would obviously reveal secret areas or other spoilers. With any luck Dyson will release player versions of each map for us to print or use digitally.