My experience since the 2.8 update has been identical to your description. I am also running on a Mac Pro, and it was suggested that I change my system preferences:
Open 'System Preferences' -> 'Network' -> 'Advanced...' -> 'DNS'
Click "+" and type 8.8.8.8
Click "+" and type 8.8.4.4
Close 'System Preferences'
Restart Second Life
Unfortunately, this fix did not improve things for me. Any other Mac users having issues with SL Viewer 2.8?