I'm not sure that assumption is necessarily right. As a general guide, on a cheap (sub $200) modern FPGA I can clock an RV64 core at 50-100 MHz. As you spend more on the FPGA, you can get higher clock rates and/or more cores. Also it should be possible to clock 32 bit cores higher (perhaps much higher) because there will be fewer data paths for internal routing to skew. On the other hand, modern RISC architectures are designed for this, whereas old 68k architectures may not be.