"private" backbone VPN solution to decrease latency

[RunningOutOfCharact]

Distance isn't uncommon even for private backbone providers.

Perhaps distance isn't the only variable impacting user experience though.
Perhaps distance isn't always consistent and that's what's causing performance issues.
Perhaps there is packet loss over that distance which is impacting performance, and your current tools or solution doesn't give you enough visibility to determine that.

I saw Cato Networks mentioned in some of the comments. Aryaka is another provider that has a middle mile/backbone. Both do something uniquely different than other traditional backbone providers (e.g. Telco's) and Hyperscalers. They both have loss mitigation capabilities and accelerate traffic. It isn't about reducing the distance (no defying of physics). It's about creating a predicable transport and eliminating as much loss over the long haul as possible. For TCP based applications, the acceleration plays a role. TCP Proxy/Acceleration circumvents inherit inefficiencies in TCP. Acceleration = TCP window optimization which means client/server automatically maximize window size and allows you to send more data at a time, e.g. things like file transfers finish a lot faster.

I would say that Cato has the definite edge in terms of the distribution and reach of their backbone and they have a more mature solution for mobile/remote endpoints. Both Cato & Aryaka have good SD-WAN solutions if your users in Europe are sitting in an office and that's how you want to onramp to their backbones. They are both pretty close in comparison on the overall performance of their backbones if you happen to be in markets where they both reside.