Hmmm.... this might be an artifact of the python implementation?

The insert speed should be roughly constant until the filter begins moving items into their alternate buckets(when one of the two buckets for that item is full). In my implementation this doesn't usually happen until well over 90% capacity.

Insert speed will slow slightly as the filters fills up if the implementation uses a shortcut to determine the first empty bucket position by just checking if it's zero. Conceptually, if a bucket is empty you only need to check the first position before the insert. When the filter is half full you need to check ~%50 of bucket positions before an insert. In practice the bucket operations are so fast and buckets only hold 4 items each, I haven't noticed a difference.