this is fixed in 1.21.100 for me
i'm having the same issue.. even with 90 threads allocated, render distance at 5, RTX off, All settings on low. once someone enters the nether… my server fans spin up like there is VERY HIGH usage but it still takes like 1 minute to generate chunks.. once everyone is back in the overworld.. fans spin down and cpu usage goes back down and gameplay becomes normal again. seems like nether generation is un-optimized completely making the nether unplayable.
this is fixed in 1.21.100 for me