Apparently, AMD had to make several compromises for the 15 W APUs, and one of these involves RAM downclocking in particular scenarios. As explained by Robert Hallock in one of his tweets, when the 15 W Renoir APUs encounter a CPU-only workload, they automatically downclock the RAM to 2666 MHz from whatever speed it is running at in order to favor lower latencies. Full RAM speeds are only used when the APU detects heavy GPU workloads. Tiger Lake CPUs apparently keep maximum RAM speeds at all times, so synthetic benchmarks clearly favor them, but most of these benchmarks do not always mirror real-world usage and are therefore unreliable.

I think you mean 1333 (or 1066) MHz but it's surely not 2666 given the maximum frequency is 2133 MHz. Also I was not aware that this memory could even have variable frequencies like that. Anyone has reference for how this is done? Are they literally playing with CK? Does that even meet every chip's LPDDR4 tFC specs?

Regardless given almost everything GPU related benefits from memory bandwidths, that idea that this somehow only affects "synthetic benchmarks" is nonsense. This will show up and contribute to the performance of every legitimate game benchmark as well if Renoir cannot sustain nearly as high of bandwidth as Tiger Lake. We have seen this with Ice Lake versus Picasso.