From a recent Gmail -> Fastmail switcher, who imported all emails from Gmail
- Fastmail's UI is far faster to use, viewing emails is instantaneous, loading a folder with 30,000 emails takes a second at most. They have infinite scrolling with native scrollbars, and it doesn't slow down as you scroll. It does break momentum scrolling sometimes.
- I'm not too sure about search compared to all the Gmail filters, but filters are run via sieve. A real power user can edit the raw sieve commands if they like, but there's a UI on top that's powerful enough for most uses. There's no "Filter messages like these" though, that was handy.
- Turning off Chat/Hangout did make Gmail use less RAM and start faster, but didn't improve load times once it was active.
- Switching was fairly easy for me, since I use my own domain. Fastmail imported my old Gmail data via IMAP and I just switched MXs.
- With you on Google Suggest. Fastmail have had their share of problems, but they report them quickly and write up what went wrong.