Since Reddit updated it look & feel, scrolling is slow with epiphany (worst with some zoom level). Tested with Safari, it's not as fast as Firefox but a lot faster than WebKitGTK Using 2.20.2 version. To get the new UI in reddit.com, seems you need to be logged in.
Yes, this is annoying....
Looks related to JavaScript. Blocking JS from https://www.redditstatic.com/ in Eolie make scrolling smooth again. But it breaks infinite scroll feature in Reddit.
I'm very excited to say that this problem can no longer be observed after https://github.com/WebKit/WebKit/commit/06b9ec1834d27a4fb8a324e29908ed274305620a
I'm going to go ahead and close this. Cedric, please reopen if you can still reproduce with 2.39.91 (not yet released) or newer.