One of the first things I do when choosing a new provider or proxies is run my nodejs script which will test it. It test various metrics such as speed, geo location lookup from the ip address.
Some proxy types are sticky for 90 seconds or so, so you’ll need to wait for the next IP address to test. Others might change for every request so you can test them very quickly.
Concurrent requests. Some providers allow concurrent requests, so you can run multiple requests at the exact same time. These type typically have higher costs per gigabyte.