My guess would be the speed and aggressiveness of the grind depends on the condition of the track. The highspeed is possibly just a general maintenance/speed hone type process while the other is more intensive.
When you block a person, they can no longer invite you to a private message or post to your profile wall. Replies and comments they make will be collapsed/hidden by default. Finally, you'll never receive email notifications about content they create or likes they designate for your content.
Note: if you proceed, you will no longer be following .