In UART mode it operates by count source divided by 16, and so when the count source=f1, BRG=0, and CPU clock =24MHz (notes), it becomes 1.5Mbps at its fastest.
The formula is shown below:
f(XIN) / [(BRG+1) x Divided-by-16 x Count source dividing]
In addition, please meet the input setup time and the input hold time.
Notes: Since the maximum frequency of the CPU clock differs among MCUs, please refer to each data sheet.