So I want to try TES4LODGEN to make elsweyr look nicer. Will this slow my game down in any noticable way?
I use it
I use LODgen, but now have had to turn View Distant Land off in Cyrodiil as well as Vvardenfell for the same reasons. With FCOM installed there are way too many creatures & NPC's, just like Morrowind.
When I had View Distant Lands ON, LODgen gave mixed results, depending on the Quality of the meshes. For instance, when viewing the Imperial City from a distance it was terrible. As I got farther away, some things turned black while others disappeared. So, as I headed away, I could see Black spires floating in the air over where the city should be.
As for whether it will slow things down, I really can't say one way or the other. To me, it just is what it is.

i think
if u use the lodgen with vanilla lods than there is no difference and i believe i read somwhere actually improves their quality slightly and may improve performance. i think it depends on your comps specs. however, the lodgen readme actually says all this but as you are supposed to use 1 of the various view when distant mods. any1 of these will reduce fps and performance. however i find that RAEVWD mod is the best in my opinion for quality and performance again tho i believe all this is said in the readme 4 the gen.