Bit of an idle question. Experimenting with an hAP ac lite and carried out a speed test. Laptop hard wired to router is getting 85Mbps download speed (internet backbone is 220Mbps and rest of network is Gbe). I appreciate it’s not going to go over 100Mbps as that’s the limit of the Ethernet ports on this router but is 15Mbps a reasonable overhead to loose? Haven’t a clue what overhead routing involves. Laptop connected directly to the backbone gets 180Mbps which isn’t surprising considering the Virgin Media link is often a bit busy in the evening.
PS. Really wish the hAPs had at least one gigabit uplink port. Means we could take real advantage of Wi-Fi 5.
PPS. Also appreciate the hAP ac lite doesn’t have the fastest of CPUs either 
Its not just the router - remember that those speedtests generally show your average speed over the span of the test, so if you took a few seconds to ramp up to 100mbps, then the few seconds at lower speeds are then factored into your average.
Along with that you have overhead in whatever protocol the speedtest you are using uses. Such as HTTP/S, multi-stream (each extra stream adds a bit of overhead along with it too), if they are fresh connections, WSS connections etc. I’d say a consistant 85/100 on a HTTP speedtest is likely to be fine. If you chuck a torrent or something at it with good proper multiple peers and some time to utilise the link, you would be seeing the 95-100mbps on the interface in RouterOS.