Neo, actually the slowest Intel processor was the 4004, a 4 bit 4k address space CPU. Still, according to BillG, Windows 9x is perfectly usable on it.
Everything comes from this - the 4040 is more like a real MCU, the 8008 is an 8 bit version, and the 8080 (the basis for CP/M) is one of the classic 8 bit micros. (The Z80 is an extended 8080).
IMO the 8086 was actually designed to be a multitasking 8080, probably for a Multitasking version of CP/M. Some of the design of the 8008 especially is still visible today ; though the registers BC,DE and HL were renamed BX CX and DX
Possibly the stupidest piece of design is the x86 ; which had a sort of multitasking mode as well as the 8086 mode. Unfortunately Intel did not put in proper instructions to switch between the two.....