Roblox Renderer: DX11
These two are the least efficient combination of settings. Try Vulkan renderer, OpenGL renderer or DX11 with DXVK enabled.
EDIT: Also disabling FPS unlocker may make a difference but I doubt it.
Please send garuda-inxi so I clearly know your specs.
While ingame, if you hit Shift+F5 you will see your FPS. When you feel stutter if your FPS stays same, the stutter may be caused by window manager / desktop environment and going fullscreen with F11 may fix it.
Shift+F2 will give more information including renderer.
I tested in 2 games, one of which is mine, everything was green except for network receive, that means it's my fiber optic connection's fault.
In untitled tag game [beta] the lag mainly happened in the lobby map (where people pick the maps) and randomly in game where all numbers would be red for like 10 seconds, i'd have 2fps and in latter game i got around 30fps most of the time. in the former game around 60 (this is without the unlocker)
I'm running out of ideas but your laptop may be thermal throttling while playing. Disable all performance related options in Garuda Assistant, apply, reboot and run Roblox with MangoHUD. If MangoHUD doesn't work, use a monitoring app like KDE System Monitor or put widgets on your desktop.
If CPU reaches above 85 degrees celsius, it may be throttling.