I wouldn't say that at all! Both WordPress and Gallery can be very memory and CPU intensive depending upon how they are configured!Tracking down exactly what is causing poor performance for a WordPress or Gallery site involves looking at the total picture (MySQL access, filer performance, server loading, concurrent connections on the server, code efficiency, javacript usage, remote content/ad fetching, embedded "offsite" web services, memory usage, CPU resources, image usage, network latency, etc, etc. etc.)
One of the biggest issues with WordPress performance is some people's tendency to heavily load it with plugins of varying code quality, and the sheer number of plugins (or even a single sloppily coded one!) can consume a lot of memory!
Gallery, particularly when resizing large collections or generating thumbnails, can also be a huge memory/cpu hog.
I'd go so far as to say that in the majority of the cases where users are experiencing less than acceptable performance on personal blogs/galleries that are not very heavily visited (those without lots of hits/visitors) these issues are more likely to be the cause of slow performance than the distributed MySQL or a slow filer, and a DH PS might make a very big difference.