If you mean OSX do this:

- Open Mail, roughly time how long it takes from left pane click to message fully rendered on the right (~2-3 seconds)

- Open Dropbox (fast before it goes away!) and do the same (~250ms)

I get roughly 100-150 emails overnight that I go through in the morning. At 2-3 seconds PER THREAD render this is insanely unproductive use of my time.

Every time I open OSX Mail I have this exact process in my head and I promptly close it.

Not sure what's happening with your setup. For me it's nearly instant. Some long long threads or heavy HTML can be up to about 800ms.

I'm also talking about my iOS experience. There are some perf issues that would be nice, but for me that's mostly in search and initial sync than stuff that's already downloaded.

The inbox-like features ARE the fundamenatal UX/UI. I don't want a mail client, I want a gmail client, and IMAP/POP3 are terrible replacements.