Each square of difficult terrain entered costs 5 more feet of movement. If I have a speed of 25 in difficult terrain and I take a Stride action I can move two squares straight, with five feet move not used. Or I could move diagonally two squares, as the difficult terrain modifier is not increased on a diagonal, still being five feet per square. I then take another Stride action, getting the same results. I have now moved 20 feet in a straight line or 30 feet on a diagonal. I do it again and I have moved 30 feet in a straight line, or 45 feet diagonally.
If this is true (I may well be missing something) moving diagonally through difficult terrain is the way to go if your speed ends in 5. How would this affect Sudden Charge with a 25 speed in difficult terrain. Would you move only up to 20 feet straight, but up to 30 feet diagonally?
These are my late night musings after a fun SFS scenario.
You don't have any extra movement during diagonal movement through difficult terrain. All of your movement is used during that stride, and can't be carried over into another stride as far as I know.
Square 1 counts as 10 feet of movement, 5 base then +5 for difficult, leaving you with 15 left to stride.
Square 2 counts as 15 feet of movement, 10 base (with diagonal movement penalty) then +5 for difficult terrain, leaving you with 0.
So either way, you are only moving 10 total feet in a single stride action in any direction.
You count diagonal squares traveled through the end of your turn, so even if you moved say 1 square diagonally then 1 "straight" on a stride, then alternated with your other strides, you would still be suffering the same penalties, just spread out.
I may just not be catching what you are trying to say.
While those two squares of diagonal movement might count as 15 feet, you still moved just two squares, and if you actually trace those two movements (one on the diagonal, one "straight"), you'll notice the difference is fairly negligible. You've moved the same distance on the x axis, just adding that you also have moved to the side. If your enemies are to the side, great, diagonal movement is suddenly slightly more effective than straight forward, instead of slightly worse, but if your enemies are straight ahead, there's no use going on the diagonal. Don't overthink it and go toward the enemy. As for sudden charge, since the Stride action works the same as usual, yes, you'd be moving in the ways you have described.