We'd really like to get to the bottom of this, but we don't have enough information yet to know what might be happening (since we can't reproduce the problem here). What would really help would be to measure where the CPU is going, which you can do by sampling the process; that should tell us exactly where the performance problem is.
Normally I would suggest that you sample the process using Activity Monitor (in /Applications/Utilities), but unfortunately it only samples for a few seconds and that might not be long enough for you to switch back to OmniPlan and conclusively demonstrate the behavior.
So instead, in this case I'd recommend using Terminal (which you'll also find in /Applications/Utilities), where you'll run this UNIX command:
sample OmniPlan 15
This will give you 15 seconds to switch back to OmniPlan and start editing those task names. Please continue to edit until the sample completes--it's amazing how long 15 seconds can seem! When the sample does finish, it will tell you that it put the sample report in /tmp by reporting something like this:
Sample analysis of process 9634 written to file /tmp/OmniPlan_9634.sample.txt
Unfortunately, the /tmp folder isn't usually visible from the Finder. You can use "Go to Folder..." in Finder's Go menu, but since you're in Terminal anyway you can also just enter the UNIX command "open /tmp".
From the /tmp folder you can access the sample report, which you're welcome to email to us at email@example.com
or to post here.
Thanks for your help!