We have noted rare problems for routers flashed with Purple WiFi Firmware interacting with Draytek Routers where the Draytek is acting as the Internet Gateway.
This manifests itself as an inexplicably slow connection where the Internet connection is generally good.
From testing, this appears to stem from the way that certain (older) iterations of the Draytek Firmware handle IPv6 traffic; turning off the use of IPv6 appears to resolve this (the connection through the Purple WiFi router is no longer inexplicably slow).
To test for this you might turn off (disable) IPv6 on the LAN
(sample here showing Draytek 2830 > LAN > IPv6 settings)
and if test to see if this seems to show an improved connection.
If you are affected by this problem the solution would be to either turn off the IPv6 permanently or perhaps to update the Draytek firmware.