we test on macs, windows and linux laptops, it is very surprising that drawing 1 rectangle is painfully slow.
Sometimes it happens when your browser does not enable hardware acceleration or when your Linux distro does not know how to switch to the discrete GPU.
We won't be able to tell without getting more of your hw specs and debug information, feel free to reach out to the Figma support or email me at skim@figma.com - this is exactly the type of issues that elud us when looking at prod metrics in aggregation.