Fps Spikes?

So ive been trying to fix this for the past 2 weeks but cant seem to figure out what is causing it. In some areas around the map I get extreme lag spikes dropping from around 300-500 fps to 20-30. In F3 + Shift whenever a lag spike hits the graph jumps up in tick percentage. These lag spikes are only in some areas, the worst being sky islands. I have tried playing with optifine, without optifine, and without mods at all but it all has the same result. I dont have any fps problems in any other games/servers besides this one. Does anyone know what could be causing this?

Extra info:
- Running a gtx 1050 ti + i5 8300H
- 4GB dedicated to minecraft
- Playing with optifine + wynntills
- 8 Render distance
- Most settings set to low on optifine

Heres what the lag spikes look like. The fps is fine while in rodoroc, but right when entering sky islands it just gets nuked:

 

Usually I'd say the issue is with jvm, but knowing this cpu, I can say it's due to thermal throttling with 95% confidence. I'd highly recommend undervolting it as much as you can if you haven't done so already, and underclocking it to 3,2-3,4ghz. Anything between the default 3,6 and 4 is a death sentence. Trust me, it'll make a large difference.

But more importantly, lock your framerate, for the love of god, you're on a gaming laptop, what'd you expect

 

I have already undervolted my laptop and the temperatures of 65-75c are just fine. My CPU doesnt start thermal throttling until about 90c. I run every single game at unlocked fps with no stutter/lag and no thermal throttling so that shouldnt be the issue. As mentioned, it only happens on this server exclusively. Also, capping the fps does not fix the issue.

Thanks for the suggestions though!

 

Your IQ is -30, just saying.

I have a gaming laptop way less powerful than him and I don't have any problems except on this server as well. Most other games work well, with the graphics set at most low settings obviously.

 

Also what do you mean by this? Could try it to see if it does anything

 

update: I looked up jvm argument fixes for lag spikes and updated it on my installation and it seems like all the lag spikes are gone. Thanks for the idea!!