Hmm. I haven't perceived any slow down. Especially not like others here are saying. Gmail still seems pretty quick to me.
Are you using a computer from the last 4 years and sitting one mile away from where the app was created? Conditions aren't the same everywhere, and you can look to see how much memory it's using, which is a lot, and it's unconcerned with reliability and redundancy in communication with the server.
It's fast for me too, apart from the one to two second load time for opening an email, inbox, filter, spam boxes etc.
What kind of connection do you have? Because I live in a rural area, have slow internet connection and the only site I’m having difficulties navigating is Gmail.