First I'd like to recommend/request two things regarding zoom:

• An option to fit the smallest side of a picture to the size of the display. Basically, enlarge or reduce so that the whole screen is filled and the picture can only be scrolled on its longest side. I hope you won't think this is just power-user cruft - with all the pictures in 4:3 and 16:9, and all the displays in 5:4 and 16:10, there would be many possible uses for it.
• An option to smooth (preferably aggressively) on upscale. I frequently find the 150% zoom useful, but the jaggies are really strong at this specific scale.

And a performance issue problem you probably know about, but which I didn't find ITT: when a folder contains enough image files, the first picture takes a long time to load, and after a deletion or move, going two pictures forward takes just as long (since the 1st is cached).
In my case folders with 3000 pictures are not unusual at all. In such folders each reload takes 6 seconds on a recent Mac Pro: that's difficult to use it for fast manual batch selection of pictures. It's frustrating since the shortcut system, the copy/move shelf, and the "skip 100" options otherwise make Xee so suitable for this kind of heavy lifting.