> seems like x86 and the major 8bit cpu's had the same speed, pondering in this might be a remnant from the 4-bit ALU times.

I think that era of CPUs used a single circuit capable of doing add, sub, xor etc. They'd have 8 of them and the signals propagate through them in a row. I think this page explains the situation on the 6502: https://c74project.com/card-b-alu-cu/

And this one for the ARM 1: https://daveshacks.blogspot.com/2015/12/inside-alu-of-armv1-...

But I'm a software engineer speculating about how hardware works. You might want to ask a hardware engineer instead.

Nope.

In any ALU the speed is determined by the slowest operation, so XOR is never faster. It does not matter which is the width of the ALU, all that matters is that an ALU does many kinds of operations, including XOR and subtraction, where the operation done by an ALU is selected by some control bits.

I have explained in another comment that the only CPUs where XOR can be faster than subtraction are the so-called superpipelined CPUs. Superpipelined CPUs have been made only after 1990 and there were very few such CPUs. Even if in superpipelined CPUs it is possible for XOR to be faster than subtraction, it is very unlikely that this feature has been implemented in anyone of the few superpipelined CPU models that have ever been made, because it would not have been worthwhile.

For general-purpose computers, there have never been "4-bit ALU times".

The first monolithic general-purpose processor was Intel 8008 (i.e. the monolithic version of Datapoint 2200), with an 8-bit ISA.

Intel claims that Intel 4004 was the first "microprocessor" (in order to move its priority earlier by one year), but that was not a processor for a general-purpose computer, but a calculator IC. Its only historical relevance for the history of personal computers is that the Intel team which designed 4004 gained a lot of experience with it and they established a logic design methodology with PMOS transistors, which they used for designing the Intel 8008 processor.

Intel 4004, its successors and similar 4-bit processors introduced later by Rockwell, TI and others, were suitable only for calculators or for industrial controllers, never for general-purpose computers.

The first computers with monolithic processors, a.k.a. microcomputers, used 8-bit processors, and then 16-bit processors, and so on.

For cost reduction, it is possible for an 8-bit ISA to use a 4-bit ALU or even just a serial 1-bit ALU, but this is transparent for the programmer and for general-purpose computers there never were 4-bit instruction sets.