How to make MDADM not consume so much I/O bandwidth during a reshape?

My Plex server is running Debian 10 and I'm using mdadm for RAID 6. I recently added 2 more drives to the array and during the reshape, when I was watching a movie via my Plex client, streaming would frequently pause. Some times Plex would just stop playing back and give me an error. Under normal circumstances, my RAID array can sustain 500MB/s read/write. When I add a drive or 2 and mdadm is doing a reshape, that speed drops like a rock, which is understandable.

But, is there a command(s) I can issue to mdadm while it's adding drives to say..."hey, if something needs to read data from the array....slow down on the reshape"?