Very bad WLAN performance

Question

I have Sophos UTM and two AP15 access points. 
 The best I am able to achieve are some 100mbit or less. Nowhere near 300mbit I should be getting. Measured on a Win10 copying big file over SMB. 
 I have a single SSID/Network, and as far as I can say everything set up standard. 
 Ethernet connection is at full speed.  UTM is latest update. 
 Am I missing something?

SaschaParis · Answer

That values sounds for a n connection reasonable. A good read to phy (connection rate) vs. real transfer rate (data throughput) 
 
 5-6 MB/s is a good value for a realworld n connection. 
 WIFI is a) a halfduplex connection and b) over a shared media (air) with c) depending on mcs and error correction noteworthy overhead. 
 Over the day throughput may be even lower, at 3 a.m. when air is really quiet it might go up to 8..10 MBytes/s in best case. 
 Remember the old 10 HDX coax cabling? The more clients were talking, the slower single client throughput became....*sigh*, good old times ;o) Shared media access...