Apple's e-cores are actually (relatively speaking) much better than Intel's. From memory, Apple gets about the same performance (actually higher in single-threaded, but same MT performance) out of about 2/3rds the transistor count as Intel. Note that this is invariant of node - obviously Blizzard cores are physically much smaller than Gracemont, but they are
more smaller than you get out of a node shrink alone, apple is doing more with
actually less transistors than Intel.
Since the Intel e-cores still have a relatively wide decoder, e-core designs may be the part where the bill comes due for x86 in terms of decoder complexity. Sure it's only 3% of a performance core, but if you cut the performance core in half then now they're 6%. And the decoder doesn't shrink that much, Gracemont has a 3-wide decoder vs 4-wide on Golden Cove, and you still have to have the same amount of instruction cache (instruction cache hit rate depends on the amount of "hot code", and programs don't get smaller just because you run them on e-cores). A lot of the x86 "tricks" to keep the cores fed don't scale down much/any.
edit:
Intel Golden Cove: 7.04mm^2 with L2, 5.55mm^2 w/o L2
Intel Gracemont: 2.2mm^2 with L2, 1.7 mm^2 w/o L2
Apple Avalanche: 2.55mm^2. (I believe these are both w/o cache)
Apple Blizzard: 0.69mm^2 (nice)
Note that N7 to N5 has roughly 1.6x logic density scaling - so a Blizzard core would be 1.24mm^2 or roughly 73% of the transistor count of Gracemont for equivalent performance! For the p-cores the number is 82%.
This is one of the reasons I feel Apple is so far ahead. It's not about raw performance, or even efficiency, it's the fact that Apple is winning on both those metrics while using 2/3rds the transistors. It's not just "apple throwing transistors at the problem", which of course they are, but just they're starting from a much better baseline such that they can afford to throw those transistors around. The higher transistor count in total is coming from the GPU, the cores themselves Apple is actually much more efficient (perf-per-transistor) than x86 competitors.