Getting low mbps from my AC1200 Range Extender

Let's set some expectations about extender performance.  In extender mode (i.e. fully wireless operation), an extender must shuffle traffic back and forth between clients and the router.  If both the clients and router are operating on the same band, then there will be a minimum 50% speed drop.  This is with client sitting next to the extender.  It will get worse as the client moves farther away.  Now, let's explore options.

The best way to use an extender is to operate it in Access Point mode.  The extender must be wired to the router, which will enable the extender to dedicate its Wi-Fi radios to servicing clients instead of shuffling traffic between clients and the router.  If running an Ethernet cable is not practical, then Powerline (networking over A/C wiring), MoCA or HPNA (networking over coax) are alternatives.  Ethernet is vastly preferred due to consistent and reliable performance, whereas the alternatives are less so.

If this is not an option, then consider enabling FastLane on the EX6200.  FastLane dedicates one Wi-Fi band to talk to the router and the other band to talk to clients.  This will avoid the 50% speed penalty I mentioned above.  Since you presumably have 2.4 GHz clients, you would dedicate the 5 GHz band to talk to the router.  Your router must support 5 GHz in order for this to work.

If your router doesn't support 5 GHz, then try one or more of the following:

1. Log into the EX6200 and make sure the Wi-Fi transmit power is at 100% (look under Do More > WiFi Coverage)
2. Try changing the router's Wi-Fi channel.  Channel 1 should have the best range, so try it first.  But radio interference, especially from nearby Wi-Fi networks may result in another channel offering better performance.
3. Place the router as close as possible to the basement ceiling.