I have a TP-Link RE215 extender paired with a TP-Link AX1500 (in AP mode).  The AX1500 has 2.4ghz disabled and only 5ghz enabled.  The RE215 is in "High Speed Mode" to transmit only on 2.4ghz.  So it links to the AX1500 on 5ghz exclusively, and extends on 2.4ghz exclusively.  All the lights on the RE215 are white, in range.  And when previously configured to "Auto" I am able to get > 100mbps download on the extended 5ghz band.  So it has a fast enough connection to the AX1500 on 5ghz.

The problem is that 2.4ghz is slow.  Usually around 60mbps, whether I'm standing right next to the extender, or farther away.  The upload is normal, around 110mbps.  When I had 2.4ghz enabled on the AX1500, I'm easily able to get 100mbps download from it on 2.4ghz.

I know the stock answer is, it's an extender, so it's in half-duplex and using half the bandwidth for the uplink to the main router.  But this is not the case in "High Speed Mode."  The uplink is on a separate channel and shouldn't impede the 2.4ghz speed.

So why?  Why is it roughly half the speed it should be?