It's worth noting that 1GHz x86-64 is not the same as 1GHz RISC-V or 1GHz ARM. Different CPU architectures have different instruction sets, so it might require more cycles to achieve the same result.
e.g. 1GHz x86-64 and a 2GHz ARM - The x86-64 chip has a desired instruction built in, so it is able to perform the desired calculation in 5 cycles. The ARM chip does not include the desired instruction, so it takes 50 cycles to achieve the same result. Even though the ARM chip is twice as fast, it will take ~5x longer to run the same task.
I've never used RISC-V, but I did some testing with a couple ARM CPUs and a couple x86-64 CPUs last year and the results were roughly: ARM took ~5x longer, and x86-64 used ~5x more electricity. For the chips I was using, and for the work I was doing, there wasn't any efficiency gain by switching fully to ARM.
I am super excited for RISC-V, though! I can't wait to have a RISC-V coreboot machine.