I connected my 4G smartphone as a hotspot to my GL-AR750S as a wifi repeater, over the 5GHz band.
I got 18-19 Mbps download and 9-10 Mbps upload. In comparison, the smartphone by itself got 20-22 Mbps download and 16-18 Mbps upload.
It seems upload speeds are reduced more than download speeds, but not down to only 5 Mbps.