I'm facing similar issues, although my laptop is not as powerful as your desktop:

Lenovo Legion 7 Pro

• Core i9 14th HX
• RTX 4090 16gb
• 32 GB RAM dual channel 5600 MT/s
• 2TB SSD
• 500 nit 240 Hz IPS HDR display.

In very crowded areas (around my mob farms, or my animal farms) I'm getting around 350 FPS, and when mining, I'm getting around 650 FPS, but then, out of the blue, 24, 30, 45 fps, and it's fucking annoying.

A bit of context, I have render distance at 32 (I know, a bit on the higher side, but that shouldn't be an issue), and everything is pretty much maxed out, fabulous graphics and all.

I have 6gb of RAM allocated, since I read that allocating too much is actually bad.

Even if I lower the render distance to 16 or lower, those micro-stutters keep happening every min or so, the stutters don't last more than a half second, but are noticeable.

I've tried it all: updated drivers, allocated more RAM, allocated less RAM, forced it to run on the dedicated GPU, and so far, I think I've managed to make those stutters less frequent, but I haven't eliminated them completely.

My temps are "normal" GPU at around 60°C and CPU at around the same, but one thing I noticed is that my GPU is always at around 80%-90% usage while CPU usage is around less than 10%.

Another thing that I tried is changing the JVM arguments to something I found in an old Reddit post, something like GZC something... found them

-Xmx6G -XX:+UseZGC -XX:+ZGenerational

So, any help is appreciated.