The maximum BUFR input frequency is the same as the maximum BUFIO frequency, which can be found in the device data sheet

Speed Grade Max BUFIO CLK Speed
-3 710 MHz
-2 710 MHz
-1 644 MHz

This means that a BUFR can be driven at up to 710 MHz (-3) and can output up to 300 MHz (-3) in order to meet the BUFR maximum frequency limitation.