Sorry for the long time, I just forgot this thread. 
Constraint rotations should be easier than free rotation for these angles you mentioned. Did you tried to use the constraint keys during the rotation command?

(08-14-2017, 12:51 AM)Priceless Wrote: No.I mean in older version (2.0) it is easy to rotate to an exact value such as 20degrees,30degrees,45degrees..etc by using only the mouse.but in latest version sometimes it is difficult.test it with some complex objects.How much vertex has that complex object and which processor your PC uses. I noticed something like that using an old PC with i5.
Constraint rotations should be easier than free rotation for these angles you mentioned. Did you tried to use the constraint keys during the rotation command?