Try verify the problem before jump into conclusion...
For stuttering suspect, check your response time of D3 mpq files.

In order to that (for window 7), go start>all programs>accesories>system tools>Resource Monitor. Then either at Overview/Disk tab, check Diablo III.exe and observe the response time (ms) of MPQs files. Ideal reponse time is 5ms and below. If you have way larger ms than 5ms, u shud try the flash drive method linked by Quazacolt few posts before mine. RM30 can get one already...

To TS, if you are using MS Essentials and Avira on the same OS then you may have performance issue due to conflict...

see link below for more informations...
Microsoft

AFAIK, only one antivirus software is allowed in your OS for optimum performance.

@ts, disable will not help. U nid to uninstall either one but I would suggest to uninstall both. Reinstall one later after restart.

U can further verify this if u did experience lag in applications other than D3.