Thanks for your feedback! I believe 400 items is fine. I had reports of slowness for users with several thousands of items (and the slowness should be limited to the addon itself, not to the whole Firefox). If you want, you can send me a message through https://pabuisson.wufoo.com/forms/zi5scw41v368kr/ so we can see if IMP has a possible perfs issue or if it's coming from something else.