If you mean mm/minute that’s totally fine. If you mean mm/sec, that’s way too fast for vector lines. If you mean parsecs per fortnight or miles per microsecond, those are too fast as well. (A number without units isn’t a speed, it’s just a number)
Not sure what you mean by “a problem with the step counter” - can you elaborate?
Random position errors are almost always caused by the steppers slipping because of you’ve exceeded some mechanical limit of the system. The slip won’t always happen in the same place, but it can.
It can be dependent on the order of the paths being run, and sometimes a simple order change is enough to make it go away. Have a look at this post (and the video) to see how very small motions at just the right speed and timing can cause major issues: