Abnormally high download speed to a server in the US from Russia, why?

Question

Server equipment

My client has a project on a physical server in the USA, the provider is web-hosting.com (the same NameCheap). Both of us have strange "problems" with the upload speed to this server, it is unrealistically high for me for all protocols, and it only through the browser, it reaches 100 Mbps for me, fixed by the provider.
Let me explain, in a project a typical file uploader in PHP without frills, accepts a request - gives an answer. The site works only through https. Based on it, an API was built that works through the same https, the client loads slowly through this api (10-20 Mbps), as theoretically it should be to servers in usa or eu. And through the browser, the browser, again, everything is unrealistically fast. As if the server is in Moscow time or in a neighboring house.
Explain to me, please, on the fingers, what could it be and what is it connected with? The client is trying to lay responsibility on my code, supposedly once everything is fast through the browser (not even wondering at all why such a speed is up to an ordinary "non-Google" server), but not through the api.

Answer 1

On the contrary, I have problems with this provider ... periodic --- speed is availability, and you yourself are geographically where you are. From our Siberian outback - as much as there is a ping of 224.403 ms.

Answer 2

Perhaps the client has a low Internet connection speed or his provider limits the speed of outgoing traffic.