CCC Negotiated an Extremely Low Connection Speed

I’ve just migrated from a MBP 2014 to an MBA M2 and have attempted a backup for the first time, using the same external drive as before. Everything seems to work just fine except that I am wired that "macOS has negotiated an extremely low connection speed to this device". And it does appear slow (expected completion about 13 hours rather than the usual 20 minutes I’ve previously experienced). How do I fix this? If anything things should be faster (using USB-C rather than whatever the 2014 MBP had and a much faster source).