Mountain Lion - Flash sluggish, spinning beachballs etc
In general, newer versions of operating systems (Mac OS, Windows, et al) tend to require more RAM than older versions. If you have less RAM than the OS needs, the computer will use virtual memory paging, which is very slow.

The 2010 Mac mini came with 2GB of RAM. OS 10.8 prefers 4GB or more. You should buy an 8GB RAM upgrade (2x 4GB).


http://eshop.macsales.com/shop/memory/Apple_Mac_mini/DDR3
http://www.crucial.com/store/listparts.aspx?model=Mac%20mini%20%28Intel%20Core%202%20Duo%202.66GHz%29%20DDR3%20-%20Mid%202010&amp;pl=Apple&amp;cat=RAM