8 GB is simply not enough for most modpacks, at least the 200+ mods one. I know, I've tried, although it was years ago. You run at consistently 95% GC usage and get constant, very heavy, collections because of it. Like every second you get lag spikes due to GC pressure. And because of the usage these look to be full collections, which are stop-the-world last I checked.

This was ~5 years ago, so I'm sure it's gotten better. But not that much better 8gb would cut it, either client or server side.