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Drupal Commerce refuses to flush the cache_form table when cron tasks run. I know it is most likely a known bug and it has tens of solutions. (I will probably just set cron task to flush it somewhere in the midnight every day.)
Does it speed up Drupal in any way?
It is part of a cache obviously, but I did not noticed any better performance whether the table size. More or less I experience slowdown, but it might be related to something else.
The cache_form table holds forms that have been loaded by the system for your visitors. Eg, if a person went to a form before the form has expired, it exists in the form_cache by default.
Sometimes forms can bypass the form_cache if they are required to be rebuilt on every page request, such as with Ajax requested forms, or external services.
If you have a Shopping Cart driven application the cache_form may contain stuff added to your visitors cart -- which they have not purchased yet (I'm not sure if commerce stores cart entries in a separate table), but in general be wary of clearing the form cache because some of your visitors state may be lost.
Expired forms should be cleared by the system on cron run.
