The speed of the proxies you have purchased, depend on two main factors:
Residential Proxies are "naturally" always a little bit slower compared to Datacenter or ISP proxies
Are the proxies close to your computer (server) geo-location
One example:
If you purchased Residential proxies and generate United States Residential proxies, you should make sure to run your scripts with the proxies as well on an United States server, to make sure the proxies reach the fastest speeds and have no long round trips.
Speed tests:
We have been doing speed tests and collected the following tests:
Datacenter Proxies: 10-80ms
Residential Proxies: 200-600ms