If the leaves are not floating on top of the water, but standing up above it, its likely time to divide and repot .

Mine is the same, time to divide, mine are covered in Blackfly too :o((

welcome to GoY,
yes this is the classic sign that they are over crowded.
when they go dormant in the autumn lift them and split the rhizomes. keep the younger pieces with a growing point. you can replant those and sink them back into the pond. alternatively you can do this in the spring.
the spare bits of rhizomes can be potted up and sold/given away unless you have space for more in your pond.

In the meantime "prune" by removing all the verticle leaves and removing some of the horizontal ones to allow for additional growth. It is almost August and the heat is on so it will be growing and flowering at its max rate.