Ever since the update with the flywheel render seperation thing, we've been having terrible lag on client side.
And its all players, that are experiencing this.

Odd, would you mind installing spark and doing some profiling so I can get a better idea of what's going on?

Once you have it on your client, run the command /sparkc profiler --interval 1, then wait one minute, and stop the profiler with /sparkc profiler --stop.

It will give you a link to a viewer for the profiling data. Send the link here when you get it.

Also, before the Flywheel separation update, did you have to run /create experimentalRendering off to fix performance issues?

Heres the link: https://spark.lucko.me/k4Ch6PB3CL

And no, we didnt have to type that.

Make sure you run /sparkc or /sparkclient. You accidentally profiled the server thread instead of the client thread.

https://spark.lucko.me/KVwgy8S4dY

ah yeah, here ya go

It actually looks like Flywheel is disabled. If you run /flywheel backend on what happens?

Removing optifine, to be able to type that command, made it about 2x worse.

Are you using optifine shaders? Could you post a screenshot with the F3 menu open (with Optifine and Flywheel installed)?

Im not using optifine shaders

Heres the pic with optifine and flywheel

that's really strange. you can't turn the backend on at all?

Not with optifine installed no.

Odd. when i enable (internal) shaders, i can type the command.

It doesnt change anything FPS wise tho.

I do not have this issue with Optifine G8 installed. It must be a mod choice conflict of some kind in some other mod in your pack.