Whatever you got going on, your network adapter is NOT creating any bottlenecks.
I don't know if that's fast or slow but it works fine for everything I need to do.
A few years ago, that would have been considered extremely fast. Today, not so quick. But that does NOT mean your network is slow, or slowing down.
Understand that is like saying that 2015 Car A is slow because it "only" has a top speed of 150MPH while the 2020 Car B tops out at 180MPH. What difference does it make if you never drive above 75MPH?
If you had 5 teenagers in the house, all streaming 4K videos at the same time you were trying to buy something on Amazon, your 51Mbps speed may be a problem. But if your requirements have not changed since 50Mbps was considered extremely fast, no worries.
You should probably look at your service agreement with your ISP. What are you paying for? But understand, the agreement likely says "up to" some speed.
Last, look at your router and your modem. As Tekno Venus noted, newer Wifi technologies will support faster speeds between your computer and the WAP (wireless access point - typically integrated in the "wireless router). If your router does not support at least 802.11n (the predecessor to 11AC), I would get a new router. If your new wireless adapter supports the faster speeds of 11ac, but the WAP only supports the old 11g protocols, your router is creating a bottleneck. The router should also support 1Gb (1000Mb) Ethernet (wired) connection.
As for the modem, you did not say how you connect to the Internet (cable or DSL). If cable and not at least DOCSIS 3.0, upgrade that too.
If you have a "residential gateway" device, make sure it is current too. Note a residential gateway is an integrated device that includes the router, 4-port Ethernet switch, WAP, and modem in one box. These technically are 4 discrete network devices that just happen to share a common circuit board, power supply and case. Some include a 5th device, VoIP (voice over IP) Internet phone support.
Residential gateway devices are commonly leased to customers by the ISP. I prefer and recommend buying my own. And I go for a wireless router (router, WAP and switch in one box) and separate modem. But that's me.
