Depends on the algorithm... the ones used with http typically have less overhead for decompression than compression.

If you mean code minification, that can depend, but in general with tree shaking it shouldn't be slower, typically. The computer doesn't care if a variable is aa or myDescriptiveVariableName.

In my experience things kind of balance out, and you end up with negligible compression gains after compression. On the other hand, it also affects the time it takes for the browser / js engine to parse and execute the code, which can be significant in this world of massive bundles.