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:

With the Steam app active, in the main menu, click Steam → Preferences.

On the left, click Interface.

Look for Enable GPU a  ...