I have the exact opposite effect.
Tried a speedtest to Orange, Marseille directly from 3BB and then through NordVPN connected to some French server:
3BB: 32.13 88.94
VPN: 340.60 90.77
And what is it without?
What does a local speedtest show?
Also True? What internet package?
Repeating against another server ("LaFibre.info"):
319.71 79.52
459.13 89.53
Can't make up a conclusion really.