If you were using geo-steering then you’re probably getting some benefit by steering traffic to origins which are in the most optimized geography for a particular requests. Argo could further extend/optimize by finding the fastest route to a particular origin.
Assume for a moment the user is in London and you have an origin server in Poland. If the public internet would have routed the request from our London POP to Amsterdam, then Berlin and on to Warsaw that could be the fastest route. Or it could be possible that from our London POP to our Munich POP and then to Warsaw could be faster. If the latter were faster then routing though our network using Argo could potentially improve performance.
But to your point if you weren’t using our Geo Steering to optimize traffic first then having a Portland based server in your LB pool would potentially be slower for that London user (Argo or otherwise) than the Warsaw based server would have been.