I heard this too.
Someone stated, that if two skills are connected, like for example AL and AT or Magery and EI, where you can gain one through the use of the other, then you will see slower gains. Don't know if thats true, but it would be at least logical.
Another thing is, the GGS works slower, the higher your total skills are.
Are you sure, the change you are seeing doesn't come from your skills capping at 700?