Modern shopsystems, and for the sake of this post I’m counting the dated xt:Commerce 3 as ‘modern’, are full of features. Features you most likely never use. We, for instance, don’t use Tell-a-friend (as it’s not compatible with german law) or wishlists (as we never saw a need for it).
However, chances are good that those unused features turn out to be a bottleneck when it comes to performance. Especially when they are coded by someone who has heard the word ‘MySQL’ for the first time (i.e. those guys who wrote xt:Commerce 3).
At X-Skating we recently faced the problem that the front page loaded pretty good, the page for a single product however had almost one second till time to first byte (the .700 in the screenshot is better than most other tests we ran).