Slow Wireless Connection on Windows 10

Those below are all nice tips but the problem on my Lenovo ThinkPad T490 with the SonicWall VPN software was that the DNE LightWeight filter was enabled (by the SonicWall VPN client).
This was limiting the wireless speed to 2Mb instead of 50Mb.

When I disabled it, the speed went back to 50Mb.
This prevents the SonicWall VPN client from working and you need to use an alternative one like a SSLVPN connection with the SonicWall NetExtender.

References

• https://forums.lenovo.com/t5/ThinkPad-P-and-W-Series-Mobile-Workstations/P51-Very-slow-wifi/m-p/4374392 

The other tips...

Drivers

Update the Wi-Fi drivers.

Updates

Windows Update -> Advance Options -> Delivery Optimization:
Disable: Allow downloads from other PCs.

Reserved Bandwidth

By default, Windows reserves 20% of your bandwidth for Windows Update, system apps and other purposes, leaving you with 80% bandwidth of Internet connection. If you don’t use Windows Update on a daily basis and the reservation limit slows your Internet down, then you can remove the limit by setting the value to 0.

Task Manager -> File -> Run new task: gpedit.msc (ad Administrator)
Under Computer Configuration, double-click on Administrative Templates > Network > QoS Packet Scheduler. Then double-click on Limit reservable bandwidth.
Click the Enabled option and set Bandwidth limit (%) value to 0. Then click Apply > OK.

Window Auto-Tuning

It's a feature in our Windows 10 to allow for more efficient data transfers. But it can also interfere with the network and cause loss of connectivity.
 
From the Power Shell (Admin):
> netsh interface tcp show global
> netsh int tcp set global autotuninglevel=disabled

Disable Large Send Offload

Large Send Offload(LSO) is another feature in Windows designated for better network performance. Well-intended, but then the whole thing of allowing background apps to consume large amounts of network bandwidth is the reason why our Internet speed takes a hit. To disable LSO:

Double-click on Network adapters > your_network_adapter.

Click the Advanced tab, then click Large Send Offload V2 (IPv4) and set the value to Disabled.

Specific to Lenovo ThinkPad T490:

Device Manager -> Network Adapters:
Uninstall Intel Proset Driver.