Seems to be both. One big advantage which is not visible on the M1 (which has a 128 bit wide memory), is the M1 pro, max, and ultra have 2x, 4x, and 8X the memory bandwidth. Something that's much harder on the x86-64 size, and (AFAIK) impossible on x86-64 laptop.

The latency does seem improved in the last level cache and main memory, at least compared to AMD's Ryzen 5000 series. Sadly looks like Anandtech has lost the people who did the deep dives and I can't find similar latency numbers for cache and ram for the Ryzen 7000 series.

Apple's power efficiency allows for a 512 bit wide memory system in a laptop, 4 times any x86-64 laptop. I do hope that AMD or Intel ups their game and allows for more aggressive memory systems on laptops. Only similar design I know of is the Xbox series X and PS5, which both use AMD chips with improved memory systems to keep from bottlenecking the iGPU.