They have shaved off 45 hours, not minutes.
It seems to be a whole complex of factors:
After the Space Shuttle retirement, the ISS orbit could be raised to provide better ballistic possibilities.
The new on-board computer allows to conduct ballistic calculations faster and with less dependence on ground control.
The orbit measurements precision has improved over time.
The communication has improved over time, going through the station and satellites.
There was a constant pressure from the cosmonauts themselves, because one gets very sick during the two days scheme.
While the sequence of events is now denser and the ballistic gaps are narrower, they always have a backup variant to go by 2 days scheme.
Altogether it made the new scheme possible, and they promise to bring it down to four and even two hour scheme.
This answer is based on two interviews given by Rafail Murtazin the deputy head of Energia ballistics department, and by Sergey Krikalev the head of the Yuri Gagarin Cosmonauts Training Center.