BSNL DNS SETTING will block access to some websites would i assume use the dns set by your internet service provider. so if it works well for you, continue using it. otherwise use one of the many third party options available like cloudflare, opendns, google, quad, NextDNS etc. is basically your own router. It would always be fast. If pings to it goes above 1ms, you better get a new cable.

Both of them? 50ms is quite bad for as it belongs to Google and should be peered directly. Can you send over results?

There are reasons to NOT use your ISP DNS ( is your router and your router likely has BSNL's DNS servers configured on it):
1. BSNL blocks websites using DNS
2. BSNL's DNS may not be as reliable as Google's or Cloudflare
3. BSNL's DNS caching may not be as good as others which means higher lookup times for some sites