"Why is my NAS so slow over wireless?"
Have you tried using a wired ethernet connection between MacBook, or Laptops, and the router and how does that perform?
The Synology NAS drive will almost certainly have a Gigabit connection and the MacBook (if fairly recent) will support 802.11n at 300Mbps. Does the Sky router support these protocols?
Also bear in mind that all wireless devices that connect to the router share the available bandwidth, unlike wired connections which will have the full speed provided by the network port. iPads will continually be using WiFi bandwidth even when not being actively used, which is why they tell you instantly that there are App updates available!