From my limited experience in this, military task forces usually don't take the usual convoy / shipping routes and travel much faster. From experience, atleast 16 knots during the early years but ~20 knots after 1942.
I do have a hint but the place is in Mediterranean. If you want to encounter a task force (playing GWX 2.0, May 1942), head to C064 which is north east of Tobruk. Move exactly to the center of the place where it's written C064 on the navigation map. Now with the ruler draw a straight line to the east, keeping it's origin at the dead center of C064. Now measure a ~30 degrees angle below this line (like between SE and ESE) and draw another line of 25 km length at this angle. Delete the former line. This new line is your patrol route and if you patrol it long enough while also submerged, you will have a healthy chance of encountering large task forces with capital ships too!
I recently bagged two capital ships here. Patrol it submerged at 2 knots and you can get contacts at either sides! (but only military ships)
Good luck!