Iran is tightening control of the Internet ahead of next month's presidential election, mindful of violent street protests that social networkers inspired last time around over claims of fraud, users and experts say.
The authorities deny such claims, but have not explained exactly why service has become slower.
Businesses, banks and even state organisations are not spared by the widespread disruption in the Internet, local media say.
"The Internet is in a coma," said the Ghanoon daily in a report in early this month.
"It only happens in Iran: the election comes, the Internet goes," it said, quoting a tweet in Farsi.
Facebook, Twitter, YouTube and numerous other sites, including thousands of Western ones, have been censored in Iran since massive street demonstrations that followed the re-election of President Mahmoud Ahmadinejad in 2009.
Those protests -- stifled by a heavy-handed crackdown that led to numerous arrests and even deaths -- were instigated online and observers say the authorities are choking the Internet to prevent a recurrence.
One DVD vendor, who sells illegal copies of Western movies downloaded online, said "you can forget about downloading stuff; the bandwidth drops every other minute."
A network supervisor at a major Internet service provider in Tehran said his company had been unable to address complaints about slower speeds, particularly accessing pages using the HTTPS secure communications protocol.
"Browsing (the net) is difficult due to the low speed. Even checking emails is a pain," he said.
"Sometimes, loading a secure Google page takes a few long seconds," he added.
Like others interviewed for this article, he did not want to be identified for fear of retribution.
The problem is not limited to slower speeds, but also affects what people can actually access in a country whose rulers take great care in seeking to ensure that people do not see or read things deemed to be inappropriate.
Earlier this month, an Iranian IT website reported that the last remaining software that enables users to bypass filters imposed on net traffic "has become practically inaccessible."
Among such software is the virtual private network (VPN), which lets people circumvent the filtering of websites.
VPN uses certain protocols to connect to servers outside Iran. In that way, the computer appears to be based in another country and bypasses the filters.
Blocking these protocols could theoretically contribute to slower speeds.
Use of VPN, or its sale, is illegal in Iran on the official grounds that it is insecure and allows access to material deemed as depraved, criminal or politically offensive.
Ramezanali Sobhani-Fard, head of the parliamentary communications committee, said VPN was blocked in early March, which has contributed to slowing the Internet, media reported.
He did not elaborate.
