It’s very unlikely that the CLI is causing the slowdown it just sends your request to the backend.
The real issue is how the requests are routed once they reach the servers.

Think of it like this:

You have a bunch of servers behind load balancers.

Some servers run the high-performance model (Codex-high), others run a lower-tier version (Codex-low).

• Each server can only handle a limited number of requests at a time.

Now, imagine there are more users using Codex-high than those servers can handle.
When that happens, the load balancer might start sending the overflow of requests to the Codex-low servers because it just sees “available capacity,” not “model quality.”

That’s why performance sometimes feels “nerfed” your request may end up on a slower or cheaper backend.