Roblox freezes for a few seconds then catches up

press F9 in game and watch the memory tab while it happens. if memory keeps climbing until the freeze, it's that game leaking and nothing on your side will fix it. if memory is flat and it still freezes, go back to the network side of it.

If everything else in the game keeps moving when you unfreeze, that's network, not your pc. Wifi drops cause exactly that freeze then catch up feeling. Try a cable for an evening and see if it goes away. If the whole client stalls even in an empty place, open task manager while it happens. Memory near full or the disk pinned at 100% points to the pc. Lowering the graphics slider helps the second case, not the first.