I think the market has splintered a little bit, low-latency "real-time" stuff tends to be targeted more by simpler devices, whereas the GHz SOCs and DSPs are mo

I think the market has splintered a little bit, low-latency "real-time" stuff tends to be targeted more by simpler devices, whereas the GHz SOCs and DSPs are more often used for heavy number-crunching or "less real-time" applications where they may be running a full OS. Some of the delay is probably due to hardware support for things like scheduling, multi-threading etc. As for current tech that can match the Z80, I know the MSP430 can have an interrupt latency of about 10 cycles. This can be below 1us with the kind of clock speeds many of the parts will run at. Apparently (according to the manual) an ARM Cortex M3 can enter an interrupt in 12 cycles. This can be much faster as the parts can be run above 100MHz.