I second Fastmail. GMail is 6MB now according to devtools, Fastmail is about half a meg. The latency in Gmail using any non Google browser makes it unusable.
That said I still prefer having my mail hosted by a "normal" company located in my country of residence, as it would probably be much easier for me to get hold of them legally if they e.g. decide to delete my e-mail account or revoke my access to it.
Note that these sizes depend a little on how you measure; some measurements will increase these numbers by up to a factor of two.
I think we're measuring different things. My 6MB figure comes from bytes in DevTools from visiting gmail.com to a working UI.
PS. if you do work for Fastmail, you can make a quick win by streamlining your import tools. You probably know from my MY records I came from GSuite (otherwise I can select GSuite), that means you know the server names to import from and shouldn't have to ask me.
The import tool and related things are something we’re working on overhauling right now. I haven’t been paying close attention to it (I’ve been working on work that is a precursor to service workers and offline support, which I’m stoked about), but the designs I have seen are looking good.