@Prospect - According to both its publish specs and tested specs, the routing speed (WAN > LAN) on your device is 135.6Mb/s. Are you getting about that speed MAX? If so, you're pretty much maxed out.
Your next question is going to be why you're getting higher speeds in AP mode, and the answer is that in AP mode, you're not routing, which is actually what is one of the more demanding tasks for a router to do.