My parents got a BSNL fibre connection in Bangalore recently and promised speeds were in the double digit Mbps range. However, the connection was very erratic with many sites like facebook loading ok once and then just refusing to load properly. The speed tests were all ok giving results in the 10+ Mbps range and ping and traceroute from the router also didn’t indicate any issues. Even changing the DNS servers to Google didn’t make any difference.
The problem was persistent across all our devices ranging from laptops to phones & tablets, and faced both on wifi and wired connections. Finally came across this old post recommending an update to the MTU size settings to 1460 (it was set to 1480 for me) and sure enough, things speeded up and seem to be working fine. It seems to have been caused by some changes in the BSNL server setup.
So, if you have a BSNL connection and are facing a similar issue, just go to your router connection settings and update the MTU settings to 1460. In case you would like to do some more granular troubleshooting, check out this post that lays down the steps to figure out the ideal MTU settings by trial & error.