Ok i got curious until the answer and sacrificed half a second of video time
I used a template that i found from the web (look at the attachment below).
Not quite fun to count and put those numbers manually. But there were only 40.
I made the same animation example above as a real animated project
and rendered every 10'th frame and counted how much second every one of them took to render.
There is no incremental increase between groups and makes kinda sense if this test is anywhere representative of what is going on really.
There are some fluctuations ( i used the computer for web browsing at the same time).
But three of them are where the merge nodes are overlapping in time so it might be related.
Interestingly the first group before the compute node is much less affected by the merge then the group after the compute node.
I had expected that it should be the same for the group before the compute node (hopefully not an error in the project on my side).
If this is really related to the compute node, in some situations there could be a different look then you normally expect maybe.
The first powerfractal without any other group renders in 68 seconds by the way.
Actually when you disable the other 3 groups it is 68 second. But when you delete those groups it is 67 seconds.
I tried this 3-4 times and it was every time the same.
I am testing this right now on a longer render. But not 3-4 times. Only 1 time. I am curious but not so much