Some info I found (Not my own results, so I will not stand by these claims):
While both Gutenberg and TinyMCE contribute to page load speed, Gutenberg’s impact is roughly 5% to 10% of the total page size compared to the Classic Editor.
Considering both the core files and the block library, the total weight increase due to Gutenberg is approximately 400 KB to 600 KB.
Ofc it depends on miljon things, but it is something.