https://www.apple.com/mac-pro/ is a good test case for scrolling. Already on normal machines it is not butter smooth, but on HiDPI machines it's near (or completely) unusable. Regardless of Xorg vs Wayland, Epiphany 43 vs 44 "technology preview", font scaling at 1.00 vs 1.15, and any Epiphany settings, the machines where the issue occurs an order of magnitude worse are the ones that are running a HiDPI 4K display (ex: at 2x global scale factor). On those machines, it's a 0.5 fps slide show. The issue is more pronounced when two-finger scrolling with a touchpad than when using a mouse's scrollwheel, but even the mouse method will be unusable. Firefox and Chrome are butter-smooth on that same display/machine (which sports an 8th-generation Intel Core i7 (Kabylake), 16 GB of RAM, NVMe SSD, and Intel graphics).
Created attachment 464639 [details] sysprof capture of epiphany nightly from 2023-01-24 As suggested by Christian Hergert, attached here is a sysprof capture of the flatpaked Epiphany 44 nightly trying to scroll that page up and down, lagging in various places (especially in the middle in areas where they reveal the motherboard and other components in the case). This is recorded with GNOME Shell 43.2 (Wayland version), Mutter 43.2, Mesa 22.3.2, OpenGL 4.6 / OpenGL ES 3.2 / EGL 1.5. The GPU is "Intel UHD Graphics 620 (rev 07)" accompanying an Intel Core™ i7-8550U (Kabylake). Technically GNOME Control Center's "Details" panel seems to say there are two GPUs on the system ("NV138 / Mesa Intel® UHD Graphics 620 (KBL GT2)"), but I don't think that the "NV138" is ever used anywhere (or I have no way to know if that's the case, and GPUViewer only sees the Intel GPU anyway). I haven't installed any particular drivers, this is all stock Fedora 37.
Created attachment 464640 [details] sysprof capture of epiphany 43.0 ...and for completeness' sake, here's a sysprof capture of Epiphany 43.0 too (that one is installed from the native RPM/DNF Fedora package repositories, rather than from a flatpak).
First thing I notice is the Epiphany UI process is processing the Safe Browsing database. Wouldn't be surprised if that really slows things down until it's finished. So try turning that off in the Preferences dialog, just in case it makes any difference. (This should only happen immediately after you start Epiphany, and maybe only once per day or or some schedule? But you caught it here and it's very noticeable.) The only other thing that looks suspicious to me is the web process spending 5% of time in libjpeg, which seems pretty high.
Oh and Safe Browsing is a failed feature that is limited to Tech Preview users only, so that won't be an issue for stable users.
Created attachment 464648 [details] sysprof capture of epiphany nightly from 2023-01-24 idling at the top of the page It seems even just having that tab focused and idle (not scrolling) eats the CPU...
At the risk of this being not very helpful: the web process is doing a lot of OpenGL stuff. So I don't think the page is really idle?
I can't reproduce this in MiniBrowser using GDK_SCALE=2. Have you tried with MiniBrowser, or only with epiphany?