The minimal system requirements of the library as of now.
I did some tests on an old Pentium MMX 233MHz (that's a "high end" P1) with the CPU part. When pumping instructions at peak rate (that is not calling the kernel's delay function or stalling RRPGE otherwise) barely, but the system could budge. Meanwhile by the design of course a single display layer was continuously rendered, but not displayed.
By this experiment I am pretty sure that with some polishing and using plain VGA output (so the least amount of slow Video RAM transfers are required) this system will be capable to run RRPGE reasonably well. So that's about the extreme minimum as currently visible.
(Later with x86 assembly tweaking it may be pumped up if such special need ever arises: there are possibilities for creative assembly which could help the emulator greatly)