Bro, DFC is the trump card for IR as far as increase of speed of express trains is concerned! Due to severe congestion in network, IR seldom give through green signal to SF trains because, the number of trains on the network is so high, that there is almost no slot left to accommodate the slow moving loaded freight trains (operating speed 55-75kmph), which are the main bread and butter if IR.
Freight train have a huge inertia and it take a very long time for freight time to stop, start and accelerate to its max. speed and finally clear the section for the next train. Hence to accommodate them, IR often have to force SF trains to follow a freight...
more...

train at slower speed (yellow signal) for a significant distance till a suitable cross over station is arrived at. This results in drastic reduction of avg speeds of SF trains and one of the main reason for IR giving too much slack times to SF trains.
Once DFC is completed the freight trains running on parallel conventional routes shall switch over to DFC, thereby vacating crucial slots or section capacity for running more and faster Exp.Trains.
More over elimination of lazy freight trains from conventional network will make the rail traffic faster, then SF trains shall seldom crave for green signal through out the run and there will be no need to give huge slack time and avg. speeds shall increase drastically (around 75kmph). Slower passenger trains shall be converted to EMUs /DMUs which accelerate & decelerate fast and quickly facilitate overtake by SF trains unlike the lazy freight trains.