A slow and unresponsive Steam app on Mac is a rather common issue. Symptoms are intermittent display freezing of the app, sluggish and delayed reaction to user input (i.e. clicking on items) and slow animations in the various screens. The issues may not appear straight away after starting Steam but may build up over time, causing Steam to run increasingly slower. However, the problems are only related to the Steam app, as games themselves run normally.
This is a known (but undocumented) issue with GPU acceleration, which luckily can be turned off rather easily:
Video playback (which is another setting) will still be hardware accelerated but the various screens in the Steam app (such as the Store and Library) are now running without GPU acceleration, which seem to speed things up considerably.