Slate GL-AR750S is slower than expected

My issue is as follows:

I have D-Link router with a 5G connection - via wireless my laptop receives roughly 500mbps.
I connect the D-Link to Slate via LAN and I connect to the Slate’s 5G connection with my laptop. My laptop now receives roughly 300 mbps. Why do I receive a lower speed when going through SLate if I am connected directly with LAN between Slate and D-Link?

Many thanks,

AR750S is 300Mbps 2.4G + 433 Mbps 5G.

So 300 mpbs in 5G wifi is not bad.