If you want your program to execute as fast as possible, you have to worry about the speed of light! A trace a few millimeters longer than its partner can have nanoseconds of delay which can easily corrupt your data!
And don't forget that you have to balance the physical shape and arrangement of components and traces with frequency and surrounding components otherwise you've created a transmitter spewing out noise at tens of MHz.
Or the corollary: if you aren't careful you can receive radio signals that will corrupt your data.
Oh, you think your wire can handle the 0.5A your widget needs? Let me tell you about transients that spike to tens of amps for a few hundred nanoseconds. But it's okay, that problem can be solved with a bit of trigonometry.
On the plus side, if you forget to connect your ADC to something, you now have a surprisingly decent random number source.
I love the absolute chaotic insanity of electronics. On the surface things make sense, but one level deeper and nothing makes sense. If you go further than that, at the bottom you'll find beautiful and pure physics and everything makes sense again.
I feel the same way about software. It's a hot mess, but under everything there's this little clockwork machine that simply reads some bits, then flips some other bits based on a comparison to another set of bits. There's no magic, just pure logic. I find it a very beautiful concept.
Not so fast, some alpha particles from a distant galaxy strike your memory chips and some bits flip. If the CPU gets too hot or too cold it starts misinterpreting opcodes, branches, etc.
The reality is that computers are comprised of several PCBs running with thousands of multi-GHz signals. So all of the foregoing engineering design principles had to be observed to make our systems as reliable as they are.