How accurate are testglider's score in writing and speaking?

I will attend my Toefl test this Sunday. I completed TOEFL dedicated session 2 from gregmat. Currently I am practicing from Test Glider. But I am getting 22-23 consistently in both speaking and writing.
In writing, normally I have 7-8 minutes time in my hand after finishing my writing. Still, I can’t find places for improvement.
How accurate are testglider’s score in writing and speaking?
Should I panic about my testglider’s score?
What should I do to improve my scores in these two sections?