The only way to "know" if fermentation is happening is to take a gravity reading and compare it to the OG value. If it's going down, then everything is working according to plan. If it's stable, then nothing is happening.
Bubbles are a terrible way to determine how the fermentation is going along. (Although very entertaining!) Especially true if you use plastic buckets. You could have a small air leak along the lid such that the CO2 is escaping in another spot.
Especially with the solid krausen forming, I would not even give it a second thought. All kinds of factors determine how vigorous and how quickly the yeast is going to behave.