What is so wrong about Gmail's IMAP? I use it everywhere and don't see any issues.
If I use just IMAP, why use fastmail instead of gmail?
If I use just IMAP, why use fastmail instead of gmail?
And we're really blindingly fast for a lot of stuff, particularly if you're just downloading new email. We split every mailbox in to two sets of storage - SSD for messages less than 1 week old and lower than 1Mb in size, and big slower disk for the rest. The index metadata is also on the SSDs, so most common usage patterns can be served entirely from SSD.
Those SSDs are a RAID1 pair of Intel DC3700 drives, which are enterprise drives and very fast even for SSDs. We dropped a chunk of money on moving to this setup, and it's made both IMAP and regular web usage very nice.
So basically, we're optimised for IMAP and we're optimised for speed. Every server has local storage, and the local storage is fast. It gives quite predictable performance, and very fast performance.
That's my story as both the author/maintainer of our IMAP server, and the designer of the server architecture.
That's not saying that GMail is awful, just IMAP isn't so much their focus, and they don't optimise for it.
There doesn't seem to be a published article that I can find on the exact internals, so I'm basing it off discussions on the protocol mailing lists.