The search for a more flexible alternative for iPhoto was long and hard, decent viewers for conventionally stored pictures are kind of rare for OSX (PC has IrfanView and ACDsee e.g., Linux has gThumb and Geeqie). I ended up with Graphic Converter, which I already had over ten years ago but now is truly a jewel. It can pretty much do anything regarding image organisation, viewing and editing. One can tell that it grew over the years driven by the needs of customers, and not a designer. Speed is excellent, I personally appreciate the extended (and clever) renaming possibilities, the EXIF manipulation and the fast folder browsing.