Sending requests via proxy is too slow

I bought proxies and I’m making Guzzle requests to the API via proxies. Some requests are fast, 1-3 seconds, some last 30 seconds, a minute or more.

I can't figure out why is that? Why are API requests taking so long via proxies? They are not free proxies. I tried to test by sending the same requests directly from my own server and it takes 1-2 seconds each request, every time.

Those proxies are located near my country, they are located in neighboring countries.

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