This isn't just conjecture. I've done experiments to back this up.

You can clearly see here that my belts' throughput is 4 items per second. But when I try to cram more than 4 items per second through the mergers, they take up to 5 items per second.

I decided to take it a step further. The hub accepts up to 16 belts of input, which in my case would be a maximum of 64 items per second. Yet, this screenshot shows me delivering about 80 items per second.

Here's what my setup looked like at the hub:

Do with this information what you will.