From what I've read, it looks like there is a way for a program with 'user' level permissions to figure out the memory layout of 'system' level processes when both are present in the CPU cache, and then use that as the starting point for another exploit. If you have to flush the CPU cache when switching between 'user' and 'system' permissioned processes, that's going to slow things down a bit, but it's also going to vary a lot with the particular situation. I think it's too hard at this stage to say exactly how it's going to impact usability, but I sincerely doubt it's going to be a big issue for most use cases.