Any and every VPN will reduce your download speed. Some by as much as 90%.
That said, NordVPN is, by far, one of the better VPNs in that respect - bettered only by ExpressvPN I think - with a reported impact of well under 10% in most tests. Throwing in a Fire TV Stick rather than a pc with decent hardware has an impact as well.
Couple of things to try that might help. For RangersTV choose a server as close to the UK as possible - Belgium, France, The Netherlands are favourites. If necessary play around and switch from one to another until you get the best results. Also, when viewing the stream click on the Settings icon and change it from Auto to 1080p, or even lower, until it stops buffering. I've been as low as 460p!
25Mbps should be more than enough for a trouble-free stream in all honesty.