Oh, very interesting!
I'm a little confused, though - let me restate what you're saying to make sure it's right.
1. There's no overall speed limit on MIDI messages to the softstep.
2. However, for LED messages only, if you send them too fast the machine gets confused and "leaks" messages.
3. Sending three extra, empty CC messages after LED messages only fixes this issue.
I wonder if simply waiting a little time after sending LED messages *only* would fix the issue...? Assuming that Max is using MIDI running status, 3 CC messages consume 6 bytes, at about 3K bytes/second that's 2ms.
I'm a little nervous sending out CC 0, because that's "bank select" - if this managed to "get out" and get sent to a hardware synth, it would likely change what sound it was making.
I confess to not having seen this issue in testing - but I've been more focused on the text display.
I'll have some chance to work on this today in Javascript and will report back.