For evaluating the rendering time, I usually use the "Network" tab of the Developer tool in Google Chrome (SHIFT + CTRL + I). When doing a mapserver request, you can see the time needed for each request:
To speed up the rendering, it depends on your layers and your map, but you can heavily speed up using some PROCESSING options in your LAYER, for instance PROCESSING "OVERSAMPLE_RATIO=1", PROCESSING "SCALE=a,b". You'll find some tips about raster data performance in mapserver on this page.
Of course, if your are able to better pre-process your data before rendering them with mapserver, it'll be better. For instance, if you work with vector data, make sure you have removed all unnecessary attributes and do some geometries smoothing (if you can afford it wrt your map requirements).