We use a hybrid of ref-counting for many things but Lua still needs to do sweep phases in its GC that cannot be amortized across multiple frames. While we do most of Lua's GC asynchronously some of the phases take so long that they can stall the main thread that wants to execute some bytecode.
Here's an older tweet with some examples (I'm not the one tweeting but I made the optimizations):
Here's an older tweet with some examples (I'm not the one tweeting but I made the optimizations):
https://twitter.com/sj_sinclair/status/1097920762147733506