Low FPS in after the Caves and Cliffs update using the hi-res texture pack VanillaBDCraft:

• CPU/GPU utilisation does not spike, usually below 50% each
• fps come back to normal after exiting specific area, seems unrelated with the amount of structures,
• in x32 version of the pack FPS normal, in x64 fps were in the 30s range, but came back to 60 after picking up all dropped items, in x256 fps dropped items lower fps to 5-6 and increase safter picking up dropped items, but max out around 32,
• low fps irrespectively of rendering distance (lowered 32 to 8 chunks, no change),
• In the Nether FPS around 20, with wirtually no structures added
• tested on backup of the same world no change,
• on the same seed in creative mode entering the Nether portal causes crash of the game either instantly or under a minute after.

Minecraft version: 1.17.2.0 (not sure why it is not listed to choose)
CPU i5 4670K @4.00 GHz
GPU GTX 1080
RAM: 24 GB
SSD: Samsung 860QVO

Thanks for any help.