Upgraded router resulting in slower WiFi for older devices and the easy fix

I recently upgraded my router from a Linksys WRT1900ACS to a GL.iNet Flint 4 but ran into a problem.  My old laptop which used to pull 200-300mbps from the WRT1900ACS was now only getting 50mbps!

How can this be?  Is there a problem or bug with the new router?  Is the WiFi chip in this thing just not compatible with older devices?  Would I have to keep the WRT in service for certain devices?

This was on my old Vaio F12 with an Intel Wireless AC-7260 card (or WiFi 5 if you prefer the current naming scheme).  It has dual antenna which means a max link rate of 867mbps (though the WRT was capable of 1300mbps if you had 3 antennas).

Here's the weird thing though.  The laptop connected to the Flint 4 no problem.  It showed an 867mbps link rate (which alone was suspect, being at the opposite side of the house) and a 4/5 bar signal - so it's not like the new router just had a terrible WiFi signal and that's why.

Then something in the back of my mind told me to check the WiFi security settings.  I remember back in the day that if you had TKIP selected, that it would drop the speed down to 54mbps.

It wasn't the TKIP issue, but it was something else security related.

It turns out it was down to the router running WPA 2 PSK and WPA3 SAE at the same time.  You wouldn't expect this to be a problem, an old device that pre-dates WPA3 will just connect using WPA2 right?

I put it into WPA2 mode only and it's running perfectly.  Slightly faster than the WRT, so that's a win-win.