Hmm, I can see your point, but only so far - 'existing general computing' like an old x86 laptop gathering dust doesn't have GPIO, for example. Personally I avoid Arduino because even though it's sizzling fast compared to a $15 RPi Zero W I'd much rather be able to use Python or Ruby, and have handy things like networking for api calls, ntp, etc.
Depends what kind of speed you are talking about. The Arduino boards are real time while the rpi is not. The rpi can do a larger number of calculations in total but it can not flip the gpio pin timed to a fraction of a millisecond like the arduino can because there is a kernel and scheduler in the way.
You can do bare-metal on the RPi also, you don't have to use a (non-realtime) OS like Linux which is where your latency issues come from.
You can treat the RPi as a microcontroller and program it via JTAG, etc if you like. Latency issues come from the OS, not the hardware.
yeah, you can either do baremetal arm under the official firmware (quad-core arm, up to 1.8ghz depending on the model)
or you can do baremetal VPU and skip the firmware entirely
500mhz dual-core VPU
128kb of L2 cache for both code and data
if you want dram, you need the ram drivers (only present for the VC4 line, pi0-pi3)
you could optionally rely on 1 closed stage to bring ram up, and do baremetal VPU from the start(4).elf to avoid the dram init sequence
and if you build upon rpi-open-firmware, you could get all 6 cores baremetal, running custom code
