max32660 sudden uart slowdown above 3MBaud
Surely the baudrate code in the eval kit is right?

If found this assumtion untrue for non-standard baud rates in many eval kit code bases.

48MHz / [0.91 * 16] = 3.3MHz

Most UART implement restrictions for the divider, which usually must be bigger than either 1 or 2. In the manual chapter 8.4 it is specified as &gt;=1 (one).