If I'm reading their benchmarks right, rbenv takes (mean) 239628.1ms and Frum takes 232944.6ms to install ruby, for a 2.8% speedup. It looks like a pretty new project, though, so that may just be the starting point.
I'd be more interested in a benchmark of how much slower it is than having the right version of Ruby directly in your path, or versus rbenv shims.