My Time Machine share is already on one drive (excluding parity), so I don't think that's the reason for your speed bump.
I don't know why, and maybe this is your case, the first Time Machine backup, on a brand new share, is always fast and without slowdowns.
Over time I could also learn how Time Machine does not play well with cached shares, before 6.9.1 I had these problems with the cache enabled, once I removed it, backups became fast.
Now with 6.9.1 we are in the same situation again, even without cache.
However keep us updated on the results of the subsequent backups, with and without cache.