Microsemi can use a 111MHz clock for their RISC-V implementation but they recommend a 70MHz limit on the milspec FPGAs. If you scroll down in the readme, they l

Microsemi can use a 111MHz clock for their RISC-V implementation but they recommend a 70MHz limit on the milspec FPGAs. If you scroll down in the readme, they list some other max clock rates for other chips. Also, you can usually mess around a bit with the timing by either changing the seed that the synthesizer is using to place and route or by manually placing and routing signals if you have a high LUT utilization and the tool is struggling with automatic placement.