Dear Bondhu,
You should consider the same category of trains:
Category - Rajdhani express:
NJP Patna via Katihar - by 12423 - 8 hr 15 mins
NJP...
more... Patna via Malda - by 20501 - 11 hrs 30 mins, (Distance is 160 kms extra).
Now you will say that 20501 has slack between NJP & BGP..
So even 12423 has enough slack between NJP & DNR..
12423 has
more than 2hrs for KNE-KIR (100 kms)
almost 3 hrs for BJU-DNR (110 kms)
Even 12423 can be speedup by 60-120 mins easily between NJP-DNR..
Which will make 12423 cover NJP-DNR section within 6:30-7 hrs. (hopefully NJP-PNBE VB via KIR NBJU will take the same time end to end)