@immychan From my experience it's mostly about RAM speed - L5 has LPDDR4-3200 while PinePhone has only LPDDR3-1333 which is further clocked down by stability issues (I believe most distros have it at 1104). For some workloads it's also the GPU - L5's GC7000L is much faster than Mali-400.
(technically both phones actually contain RAM that's capable of being faster than that but LPDDR4-3200 and LPDDR3-1333 is what their SoCs limit its speed to)
@immychan When it comes to the CPU, then aside of the obvious clock speed increase (1.152GHz vs. 1.5GHz), the Librem 5 SoC also has twice as large L2 cache - but I'm not sure how much of a difference it makes in practice.