Yes its true, I had observed this 3 years back when I was living with my roomies in Mumbai, I was having a prepaid connection and 2 of my friends had Airtel postpaid and they used to get 2 or 3 times more speed than me.
Now in my hometown I struggled with mobile data speeds when my Airtel fiber went down, recently last month I ported my number from Airtel Prepaid to Postpaid and the speed increase has been 3-4 times, earlier I used to hardly get 1-2 Mbps download speeds but now the speeds range from 20-40 Mbps but it is only true for download speeds, upload speeds are still the same, and I hae been getting 4G+ in network tab while in prepaid it used to be just 4G.
But on the contrary I am obsering more call drops now, not sure the reason but someone calls me and it disconnects in 2 seconds, and I am not even able to pick the calls in those 2 seconds, and to the caller the message comes as call ended.