Yes, you're right and that's what I discuss in the last paragraph of my comment. And, yes, E-cores are rough descendants of some processors that were sometimes called Atom. Using Intel marketing names is fraught with peril, though (see, Celeron) as "Atom" has referred to many conceptually different microarchitectures over time; the modern E-Core is of no relation to the original in-order "Atom" processor many remember.

I've had the same experiences as you with Intel mixed-core desktop parts. They're incredibly difficult to optimize for due to the heterogeneous core mixture, whereas AMD mobile parts are generally more reasonable (you're on a slow core or a fast core, basically), and AMD never made a mixed-core desktop part.

However, Intel server parts several years ago switched to E-core only or P-core only, so all of the heterogeneous core mixture issues aren't a thing - you basically have two separate processor generations being sold at once, which isn't particularly surprising or uncommon.

With AMD server processor families (linked in my comment), depending on the part's density you get either "slow" or "fast" cores and either "wide" or "narrow" units, so you do still have to think about things a little bit there too.

Where Intel really screwed up in general, microarchitecture differences aside, is AVX512. That's the wrench that prevents the same compiled code from running across most Intel parts - they just couldn't decide what they wanted to do with it, whereas AMD just chose to support it and stick with it, even though the throughput for the wide instructions is wildly different between processors.

I've never understood why Intel didn't just soft-disable AVX512 on P-cores until the OS writes a value to some MSR that means "I understand that only some cores have AVX512".

From the OS side, the change to support it is pretty simple. On the first #UD trap caused by an AVX512 instruction being missing, pin the process to just the P-cores and end the process's timeslice.