After upgrading to the latest firmware ( 1.2.2 Build 20231226 Rel. 74361 ), this is my configuration that takes the most of the PX50:

• Every PX50 should connect in line, not to to the principal.
  • Every PX50 will have a degrade speed, unfortunately.  The main PX50 speed is the one that will have the full speed provided by the ISP:  Synchronous 500Mbps.
    • The reason for this should be provided by Tp-Link.  If the connection is Wi-Fi only, this is explained by the walls in the middle. But it has the PCL connection too.  The option of having PCL only at least for testing purposes is not available.
• Rebooting (if needed)
  • Make sure you need it.  In case you do not live in a very density place (apartment building), maybe you can schedule to restart all the Decos daily or once a week.
    • In my case, I only restart the Decos PX50 if needed.  I used to scheduled them to restart daily at dawn.  The problem is that the PX50 always select channel 2 for 2.4GHz and channel 40 for 5Ghz, which are very crouded in my area.  I assume that at that time, there is no demands and then the PX50 selects these channels.  By not restarting them, the DECOS maintained other channels (not crouded) provided by the latest network optimization. 
  • Disconnect the main PX50 for 30 seconds and plug in back
  • Then perform the same operation for the nearest PX50 and then to the nearest to the second one and so on (in line).
  • Run the network optimization
• Power outage
  • Normally, the Internet speed is like chasing ants after the PX50s are back online when a power outage occurs (very slow).  I do not the cause of this. Network optimizing did not work either.
  • So, I have to restart the DECOs again by following the same steps as explained before in Rebooting (if needed), unplugging them again.

Since I could not find a TP-Link reseller to buy a fourth PX50 (only Amazon sells them in pack of three), I bougth a X60 unit (Wi-Fi) for a fourth room, and it works fine.  Not the full ISP speed, but...   The point is that for the above instructions, you have to unplug the X60 unit until all the PX50 are online (as stated by the Deco App). Failed to do so, you will not be able to optimize the network and you will always be stuck on the very crouded channels.  After restarting the PX50, then you can plug back the X60 unit.

Again, maybe Tp-link folks can say something different, but this is the recipe I use and it works fine, after months of dealing with very low speed.