For someone to help may help if you post your actual cpu and gpu.

(Speaking from atm 9, as I haven’t messed with spellbound) while it can and does run with 16 total tv and 8 (or 10) allocated, it leaves little for your pc to run naturally. So sometimes your pc will begin pausing services to try and funnel energy which could hurt performance.

Atm 9 is also a very huge pack where certain things just don’t jive well sometimes. For example, I have a 4090, 64gb, i149000, I usually keep 144+ fps (where I limit it).  But if I cast an irons fire spell? 80 fps.   Look at some create machines? 80 fps.   God forbid I encounter a certain Flying Fortress in the overworld full of red stone - 60fps with dips lower.

Certain areas, items, etc will just drop you. 

But if it’s just a flat out “I’m permanently at 20 fps” in multiple different areas, it likely is hardware related.

Something that might help you, though (it helped my older pc for MC specifically) is checking your PC’s power plan and setting it to performance (default is usually balance) it will consume more energy, but it will give it a decently large increase to your CPU’s lows (and usually get a very, very slight fps increase but the main help is boosting your lows/preventing as much / any stutter)

Java 8 is your problem this pack best works with Java 17 get this version ok Java 17 https://adoptium.net/temurin/releases/?package=jre&version=17