It takes 116 down arrow key presses to scroll exactly one page...! It would be better that one arrow key press was equivalent to 1/8th of a page down.
I'm not sure this makes sense, because it would eliminate the ability to scroll slowly with arrow keys. If you want faster scroll speed, just use the PgUp/PgDn keys or increase the key repeat rate (System Settings > Input Devices > Keyboard > Keyboard Repeat) and keep using the arrow keys.
The issue with PgUp/PgDown is that it does the entire screen and you lose surrounding context. Perhaps fast-scrolling can be done by a modifier like: `Shift-Up/Down` or `Ctrl-Up/Down`. This is similar to video players. As further justification, other PDF readers have much faster scroll rates. Okular seems to be an anomaly in this regard.
Did you try to change "Page Up/Down overlap" in "Configure → General → View Options"? This is currently capped at 50%, but if that's not enough for your use case, we might have to consider changing this. Please test and comment.
If you can provide the information requested in comment #3, please add it.