>Given a syscall that does nothing, a full round-trip under BSD would require about 40μs, whereas on a user-space Mach system it would take just under 500μs

Point of reference: I work in HFT, and it's possible to process a market data update from an exchange, reprice a complex financial instrument, decide whether to place an order, and send that order back towards the exchange all in under 5us. Another point of reference: raising one float to the power of another (a^b) could be done over a 100,000 times in the same time it takes for Mach to do a single syscall.