15,000 bytes is 3750msg /sec (as usb midi is fixed at 4 bytes per message) ,
and I determined this behaviour was happening at close to 1000 msgs/second.
also that report say its drops messages, not that the macOS stalls (which is doesn’t) besides I can clearly see on midi monitor that msgs are going thru - its pure data that has barfed.
anyway, a friend and I have determined that this “bug” was introduced in PD 0.48.1, as it was fine in PD 0.48.0 
I guess I’ll just use 0.48.0 on the macOS for running for now (but dev on 49, I can throttle down for dev purposes).
anyway my main target platform is ARM so not really an issue for me… just cost me time, trying to figure out why my patch wasn’t working correctly.