Flow does have different performance from wikitext talk pages. Some of this we may be able to improve. E.g. we improved phab:T114550 (more could perhaps be done). There is some future related work planned too, such as phab:T94029 .
However, the fact that Flow uses JavaScript normally (there is a no-JavaScript mode available) means that it's doing more work (e.g. the dynamic table of contents, etc.), which requires a bit of setup time; this also probably could be optimized, though.
I don't have the statistic you requested (page size). This is not the only factor, though (and maybe not the most important one, but it depends on your connection speed). Others are server processing time and JavaScript processing time.
The first time you load the Flow editor, it will take a couple seconds to load VisualEditor (you can also use wikitext). However, this should only happen when your browser first caches it, not every time.