Is it true that cake works much better with two cores and fq_codel with a single core?
From what I've read on this forum from people who are much more knowledgeable about sqm than me, yes, they usually recommend a dual core device or a x86 machine to take full advantage of sqm, although I'm not sure if that applies to both cake and fq_codel, or just cake.
SQM itself only needs (uses) a single core, but your router doesn't only do SQM, there's also PPPoE (if needed), NAT, routing, firewall, wireless, etc. Multicore does help a lot in this regards, reducing contention, keeping enough headroom and avoiding unecessary lags. For it to work well, especially at higher speeds, you need CPU cores that are fast enough to deal with SQM at your configured line speed (single threaded) and use the additional cores to handle the other task.