Why was TARDIS 300 years late on its way to Trenzalore?

Okay. Clara jumped on to dematerializing TARDIS, so TARDIS extended its force field to save Clara from Time Vortex. So, it needed to run at slow pace or time pace, whatever.

But, why didn't TARDIS simply open the door, let Clara in and ran with normal pace? Last I remembered, she was holding the doors.