OK, I've had mixed results with the beta versions: 2.94.15 worked for me (over 2.86); 2.94.17 over 2.94.15 was a mess, for me.
I've had to revert to 2.86 before, so excuse me if I hedge when I say that version 3.0 is working fine for me, so far.
I 'exited' Sandboxie 2.86 Control, installed version 3.0, and rebooted - no problems - at least with sandboxing the Firefox browser, and that's what's most important to me.
One thing that I know is different on my computer, versus conditions previously, is that I no longer have Spy Sweeper 5 installed. That program's latest software update was severely slowing down delete operations for me, so I just un-installed it instead of reverting to the previous build or turning-off some of the shields to see if it would improve. I purposely left it un-installed in anticipation of upgrading Sandboxie. Did that help - I can't say.
Save some of the bubbly stuff for me
WinXP Sp2, Pentium 4, 2.4 GHz, 1 GB ram