I am in India using Reliance dongle.
perhaps your server is overloaded
maybe your computer has been infected by malwares n all.
get your computer a full scan with malware/spyware scan.
also, try reseting your router , and configuring it once again.
also, maybe a newly installed appliance/device is causing interference in the surrondings
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Common problem in wireless connections.
The speed of your connection depends on the signal strength of your dongle.
Another reason can be contention.
Could be any of several reasons. As well as the two already mentioned (contention and signal strength) there's traffic shaping and fair use limits.
Traffic shaping - some categories of internet traffic are given priority over others. In simple terms: you want email to arrive promptly but if you're downloading a very large file you are expecting it to take many minutes or maybe hours so a bit slower matters less. It's possible some telecoms providers abuse this to degrade VOIP connections so you use their conventional telephone systems which generate more revenue.
Fair use - My broadband provider throttles the connection of anyone using excessive bandwidth. They try to be reasonable about it - so if you schedule large downloads after midnight when demand on the network is low there's no penalty but if you use a lot during busy daytime hours then they put a restriction on your line speed for 24 hours.
There are some scenarios related to malware activity - a hacker trying to get into your PC, a trojan utilising your bandwidth as part of a botnet.