@Ritikoberoi 

How are you connected to the Archer C20 when you run the speed test?

The spec sheet for this router shows its WAN and LAN ports are only capable of 100 Mbps.

So, even in best case you will be getting a few percent less than the 100 Mbps through the router, because there will be small losses here and there during the routing of the data.

If your client device is connected to the C20 via Ethernet cable or via the 5 GHz Wi-Fi, then I would still expect around 95 - 97 Mbps.

But if you are connected via the 2.4 GHz Wi-Fi, there is a good chance you might end up with lower speeds, especially if your client device has only one antenna.

@woozle 

i have connected through a box of fibre connection  which my local service provider have provided me and im using 5ghz connection but it says it support around 750mbps isnt that right?

@Ritikoberoi 

That "AC750" moniker is merely a marketing thing.

According to its datasheet the C20's Wi-Fi is rated 300 Mbps for the 2.4 GHz radio and 433 Mbps for the 5 GHz radio. The "AC750" is the result of 300 + 433 = 733, which is rounded up to 750.

Anyway, the Archer C20 is a very low end router. According to my favorite price comparison app the C20 is currently the cheapest dual-band router TP-Link has on offer (at least where I live). It is based on a slow single-core CPU and its Internet/WAN port is only 100 Mbps. So maybe the C20 is just not powerful enough to handle the full speed of a 100 Mbps Internet connection. 

@woozle 

ohh ok well I purchased it 2-3 years back so i think now its time to change XD anyways thanks dude for your help.