What makes an ISP proxy low latency?
Three things, in order of impact: how far the exit IP is from the target server, whether the exit sits on stable infrastructure or an end-user device, and how many hops your traffic takes before it reaches the proxy. Static ISP proxies address the middle one by design, and country allocation lets you address the first one at purchase time.
The third one is usually self-inflicted: chaining a proxy behind a VPN, or routing through a gateway in another continent before the exit, adds a round trip that no proxy quality can win back.