Updated: Comment #N
Since the introduction of the key_value_expire table @Cottser and I noticed during profiling using xhprof-kit. that each run of the profilier on a requesting a page with a form on it that the profiler would consistently get slower and slower on each bbranch run. We looked for the problem in the profiling results and it indicated a PDO execution getting worse and worse and so we tracked down the table and it turned out that key_value_expire would balloon out of control.
A temporary solution was to truncate that table after every run. See pull request #4
Provide a way to clean the expired values. Possibly other solutions.
User interface changes