That's strange. I've had the opposite experience. I was with Cyberghost and, after 3 yrs of good speeds, almost overnight it basically became so slow that it was unusable. I then tried out Surfshark and have been very happy with the speeds that I've gotten for the past year+.
I had been with Surfshark for nearly a year when everything slowed down. They could have been having temporary technical issues, of course, but it went on over a long enough period that my troubleshooting made it through multiple steps to trying a different VPN provider - so over a week, IIRC.
In the end, it depends on how your ISP peers with your VPN provider's network. VPN companies tend to host their servers on networks with cheap bandwidth, which don't necessarily have great peering with many residential ISPs.
