I use Firefox on MacOS. It is much slower than Chrome for me (and I don't tend to hold open many tabs or windows). I use Google Mail/Groups/Calendar all day long which became unusable in the last months.
Open your email in thunderbird or move your email somewhere else. This is all part of googles game. They slow down youtube in every other browser as well.
IIRC this isn't intentional, but it's not due to benevolence either: The whole custom elements fiasco (which is largely Google jumping the gun and shipping an unstandardized API that they eventually had to deprecate) means that some Google properties are still written on custom elements v0, and use a polyfill on non-Chrome.
