There are a lot of things that can affect speed. Protocol, channel, antenna, switches, ethernet cabling, interference, etc. etc. etc.
Are you connecting on 2.4ghz/5ghz radio? over what protocol? Do you see the same speed results with different devices? What are you using to test your speed? Are you getting full speed on a wired device?
Here are the supported data rates per the spec sheet of the ZF R510:
- 802.11n/ac: 6.5Mbps – 173.4Mbps (20MHz)
- 13.5Mbps – 400Mbps (40MHz)
- 29.3Mbps – 867Mbps (80MHz)
- 802.11a: 54, 48, 36, 24, 18, 12, 9 and 6Mbps
- 802.11b: 11, 5.5, 2 and 1 Mbps
- 802.11g: 54, 48, 36, 24, 18, 12, 9 and 6 Mbps
Keep in mind that the high end of those data rates are going to be under pristine conditions.