An important point about scrub planes for those not familiar with these very useful implements: they are generally used
across the grain, not
with it like 'regular' planing. Cutting across the fibres takes far less effort per kilogram of wood removed (like when using a gouge for rapid stock removal).
You usually go back & forth along the board at an angle of approximately 45 deg to the grain direction, switching the angle to left or right on alternate runs. Work on the high spots first, which you determine (& constantly check), by sighting along the surface & using winding sticks (just a fancy name for two straight pieces of wood of the same thickness placed across the board at the ends to check for twist or 'winding').
As said above, you can remove a very impressive amount of unwanted wood very quickly with any type of short, light plane set up for the specific task - I used an old #4 modified for the task for many years. Wooden scrub planes are great if you work with 'sensible' woods, the less weight you have to push around on this job, the better, but in my part of the world, wooden soles wear rather rapidly on our bone-hard hardwoods, so I use a metal-bodied version. I frequently dimension rough stock by hand rather than go to the bother of wheeling out the screaming electron-burner if there's only one or a few pieces to size, takes about the same time or less, is noise & dust-free, and a pleasant way to get some upper-body exercise.
On soft woods with docile grain, you can use your scrub plane along the grain if you just want to clean up a rough board to approximate thickness, but better to use something jack-sized, with a less aggressively-radiused blade for that purpose, it will reduce tear-out a bit, & the length will help keep the surface level. I've seen the telltale grooves of a deeply convex blade on the backboards of old pieces many a time....
Cheers,