Hi Slothmonger,
You are actually pretty correct its about 41% increase.
When you finally calculate the movement you need to normalise the velocity and then multiply by speed.
direction = velocity.normalize()
move_and_slide(direction * speed)
This would limit the velocity to a value between 0 and 1
I found this thread on reddit that may help the understanding a little better
https://www.reddit.com/r/godot/comments/10wj72a/if_i_move_diagonally_the_character_moves_faster/