My Macbook Air's connection shows only around 20 mbps when other devices are at 800. Router is definitely 3 feet away. It frequently shows "No Internet" when the wifi is working fine. How can I fix this annoying issue?

1 - Recommended settings for Wi-Fi routers and access points

2 - Added - Any Third Party Applications that will interfere with the normal operation of the OS,  is an invitation for disaster and comprising the Operating System

3 - Any VPN Installed on this problematic computer ?  Commercial VPNs   should only be used if Mandated by Corporate Head Office for working remotely. Otherwise - ones Privacy and Security touted by the VPN is suspect.

4 - Any of the below should be removed as per Developers Instructions 

This will include CleanMyMac , This will include BitDefender , 

This will included Norton Antivirus , Sophos Av Software

Intego AntiVirus, McAfee, MacKeeper, Avast AntiVirus

Ad Guard, Webroot , ESET

The The Built in Security  is all that is required.

What year Mac? Its network card's max speed is determined by the protocol it uses. Your others devices may be using a different protocol.

What does PHY Mode show when you Option click the WiFi signal strength icon?

That is a very large difference, but there are so many factors involved and proximity to the router is only one of them.

You might want to run diagnostics on your hardware: Use Apple Diagnostics to test your Mac - Apple Support

Are you saying that the devices reporting 800 MBs are connected to the same router and are also WiFi?

800 MBs on WiFi is almost impossible to get even with gig Internet connection. I've seen a guy from Germany who gets 900 MBs on his WiFi. If you're getting 800 on a wired connection that's good and is completely different than WiFi. On WiFi 300-500 is about the tops I've seen. Typically I see about 200-300 when I'm close to my router.

That aside 20 is the pits and I'd suggest that you get an app that will map the WiFi signals around your location and show you the channel everyone is using. WiFi routers typically come preprogrammed for a common channel so it's common for you and your neighbors to be on the same channel. When that happens, your router has to deal with collisions as your WiFi signals are comingled with your neighbors. When your router has to sort out your signals from your neighbors, it means a slow connection as the router has to resend data packages. There are a bunch of articles on selecting the proper channel but often if you move your WiFi channel to a one with lower use the speed will drastically improve. I use a WiFi analyzer app on my phone to figure out the channels being used around my house to move my router signals.

If you can't get high speed (at least 500 MBs) with an ethernet connection, then you must have a hardware issue. I'd suggest the following:

1 ) Swap out the RJ45 cable and run SpeedTest.

2 ) If you have another computer, run SpeedTest using the same cable and connection that you're using for the Mac.

3 ) Check the Activity Monitor to be sure the CPU load and memory used is low. I don't know what the Air reports but my MBP's CPU load is almost zero at idle.

4 ) Run a disk speed test to be sure your SSD is working properly.

5 ) You didn't mention if programs seemed to run normally on your Air.

If you can't get high speed with a cable and another computer works fine at that same location then you've obviously have an issue with your Mac. Assuming you are not under warranty then I'd suggest that you do a Time Machine backup then use Disk Utility to reformat your drive and reinstall the operating system. Before migrating over your apps check the speed and see if it is high (normal). Then if the speed is okay, migrate over your apps. If the speed then drops, one of your installed apps is slowing down the system.

Wifi Explorer in the Apple Apps Store - Paid will give an over view of your specific Signal Strength, what and surround Wifi Signals in the area.

refer to below illustration

I restarted my computer and router and it worked for me.

[Link Edited by Moderator]