Disclaimer: posting this on behalf of a buddy, so I don't know too much about the mod's internals, only what happens and when.
Tested on g4.5pre-61-g4de8851e0 x64. Can only reproduce with Vulkan. Tested with OpenGL but couldn't get it to lag. It might not be impossible, just really, really difficult to do so.
Mod author reports the same issue and according to him it has been present for quite a while. If I understood correctly, even back in 4.3.3.
There are no exact steps because it always seems to be random, but the closest I can come to those is:
1. Start a map, any map. I've had it happen on E1M1 and MAP01. Tested the most on E1M1.
2. IDFA/give all to get all weapons
3. Empty the BFG into a wall. Try shooting the listed weapons in various directions for a better chance of lag.
4. Empty a mag from the rocket launcher (slot 5).
5. Empty the entire flamethrower (slot 5). Use alt-fire as well as that seems to be a good trigger.
NOTE: If it doesn't lag, go to step 2. It's bound to lag. Haven't had to IDFA more than twice before it does.
By the time you're done, "stat gpu" should show about 30 ms for SSAO. It doesn't go up gradually. One moment it's 1 ms and the next it's 30-50 ms. This is NOT cleared when switching maps, which is the main reason I'm reporting this and not writing it down as a user error. The only way to get rid of it is to either restart the game or disable SSAO. Re-enabling it again starts lagging everything down to 4 FPS. It gets worse if you look at outdoor areas like E1M1's blue armor secret.
The mod is private, so link to DL the example will be sent to the developers via PM shortly.
NOTE: The mod is going to take a while to load. It ramps up my RAM usage to about 2 gigs before it goes down to ~800 MB. Starting a level is also going to take about 4-5 seconds.