Some days ago I tested caching on a very lightweight site running on GP. It literally did not make any difference at all, like a few milliseconds or so. And I can’t really say whether this was due to caching or just the usual difference between measurements.
I’m a little biased here, since I really love GP. But I would say with confidence that a properly made site running on GP is as fast as WordPress can be. However, even huge differences in load time between sites are normal. A Woocommerce site using sliders, a page builder, social media and so on will always be much slower compared to a GP site, that is lean and mean.