I have run into the same issue. Rather than delete everything, I created an Archive list, into which I dumped all of my non-repeating, completed tasks (completed tasks stay linked, so to include them would have grabbed every instance, not just the done ones.) After I moved all that to the archive, I then archived the archive, and RTM returned to its usual snappy pace. Now I do that every 6 months or so, and it perks things up nicely.

ranbarton, that's a good workaround, especially if you synchronize with an iPhone (and have it set to synchronize all completed tasks). Do you use a Smart List to collect all the tasks to be moved to the Archive list?

If either of you haven't done so already, would it be possible to submit a Tasks & Lists issue report via our support system?

Please select the issue that you're experiencing from the list on the left, and follow the steps shown. If your issue isn't listed, just click on "I'd like to report a new issue" on the left.

Your report should provide all the info that we need to investigate further, but we'll be in touch via email if we require any more details.

Thanks!

@andrewski - I do use a smartlist to assemble the tasks to archive. I will go submit the report now.

I had been experiencing heavy slowdown issues (and was actually considering switching to another task manager) until I read this and deleted all of my completed items.

Things have really sped up now! Thanks.

Just a reminder: if you'd rather not delete your items, you can achieve the same end by archiving your completed tasks. You'd want to move all tasks to the Archive list and then archive the Archive list. (I tip my hat to ranbarton above, who mentioned it first. :)