I ran some benchmarks, with Flash Benchmark 2008 from Snails animation. On Windows XP SP3, Firefox 3.6.8.
25.39 fps with plugin-container (oopp) turned off, 2nd round
22.9 fps with plugin-container (oopp) turned on, 2nd round
So, turning on plugin container made about a 10% slowing on that benchmark. If your system is just barely able to play HD video, or a certain resolution of video, disabling the plugin-container may give you enough of a boost to help.
For most people, reducing crashes will be worth a slight slowdown in flash, but hopefully performance can be improved in the future. Also, by making flash slower, it may be possible that the separate process for plugins is making the responsiveness of the rest of the browser better, by prioritizing the browser more over flash. If you really want the crash protection and want to experiment with improving flash performance, you could try setting the plugin-container to higher process priority - a utility like Prio - Process Priority Saver
http://www.prnwatch.com/prio.html may let you do that.