I understand, but it doesn't change my answer. Do you have support cases open for these issues? While I've worked with some customers that saw slow merge over time, this is far from universal, and it's not currently clear to me why it happens in only some cases. I have a suspicion that it is related the the cache fix which Microsoft is still working on, as far as I know, but I don't see how anything could be changed on the Veeam side to solve this. The reason I suspect this is because, in one case, we found that if we rebooted the server on the afternoon before the merge, performance would be very fast, but would again slow over the course of weeks.
I'd be happy to look at your environment and compare to the others that I've worked with that don't have issues, see if we can find anything common. If you are interested please PM me.