1st.
You need to start a brand new character to maximize the effect of the smaller saves introduced with the patches.
2nd.
You can still play your old character but will probably always have the lag when play time exceeds a couple hours, these always require full resets. To minimize these you need to turn off all hardware acceleration under Game and Video settings, sign out of PSN and limit your time in the XMB.
3rd.
Similar to tapping shift 5 times for sticky keys on the PC (Which literally slaps your computer out of similar instances) hold down the PS button as if you were turning off the system or controller and then hit circle to back out.
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I have well over 100 hours on 4 characters, they all play pretty good with the same minor hiccups that occur on all platforms. My first character just has to restart every few hours, and really I always use the first character to explore and find out what things I shouldn't screw up. I'm fully patched and everything works fine, there are definitely people that have issues.
I've tried to duplicate the problems on 5 different PS3's, and all problems seem to focus on the fact that people are stuck on their first character and refuse to create a new save. If you are adamant about keeping the same character, you are going to have to restart every few hours. Don't wait for it to death lag, just restart when things start slowing down.