I ran into an issue where completed tasks were bogging down the native task management application, Tasks, on my HTC TouchPro 2 when it filtered out the completed tasks. I use MilkSync to synchronize my tasks with RTM. I contacted support and Andrew suggested I archive the completed tasks into a new list.

What I did was create a new task called ArchivedTasks. I then ran a search on All Tasks for "completedBefore:now" and saved the search. I moved the results to the ArchivedTasks list. I then archived the new list. I ran a new search on All Tasks for "includeArchived:true completedBefore:now" and saved the search. Since both searches are saved as Smart Lists, the results will always update automatically. To get new completed tasks archived, I unarchive the list, run my Completed Before Now search and move the results to the ArchivedTasks list. Then, I re-archive the list. Again, if I want to review completed tasks I've archived, then I run my Completed and Archived search. I'll only do this process when Tasks appears to be running slowly.

Additionally, those tasks are no longer sync'd to my phone. To be safe, I run a restore from MilkSync on my phone to keep the completed tasks from loading in RTM. I don't think they would, but I don't want to mess with deleting them. Once the restore was done, the completed tasks were gone from my phone and Tasks was zippy again!