What you described is not related with the mod (mod doesn't touch anything in 3d rendering), so most likely there's just a way to much stuff on screen or objects are simply too small - fps improves when you zoom out (switch to LOD mesh - significantly less objects, much lower mesh resolution) or zoom in (fewer objects).
GPUs struggle a lot when mesh is super small/has way to many details (multiple vertices/faces per pixel), even 5090 won't help too much since calculating such small details is basically wasting GPU time - you won't see the details but GPU has to brute force it and at the end it will throw away 90% of the calculated data. That's why LOD are so important (there are only 2 level of details - full resolution and ~1/5 or lower, depending on object type).