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 why service has become slower. Businesses, banks and even state organisations are not spared by the widespread disruption to the internet, local media reports.
"The Internet is in a coma," said the Ghanoon daily in a report earlier this month.
"It only happens in Iran: the election comes, the internet goes," it said.
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. "Even loading a secure Google page can take a few seconds."
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 are virtual private networks (VPNs), which let people circumvent the filtering of websites.
VPNs use 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.
The use or sale of VPNs is illegal in Iran on the 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 VPNs were blocked in early March, which has contributed to slowing the internet, media reported.
Authorities refuse to officially confirm the new restraints, but former officials and media reports say the Supreme Council of Cyberspace ordered them .
The council, set up last year, is tasked with guarding Iranians from "dangers" on the Internet while enabling "a maximum utilisation of its opportunities". The complaints come as Iran prepares to elect its new president on June 14, but the authorities reject claims that there is any link with that and the current problems.
"Many parameters are involved in the internet's speed, but the election drawing near is not one of them," a deputy ICT minister, Ali Hakim Javadi, said. The disruptions are also linked to Iran's stated plan of rolling out a national intranet that it says will be faster, more secure and clean of "inappropriate" content, observers say.
Critics say the unfinished "National Information Network" could expose Iranians to state monitoring once operational.
Users of the state-approved VPN service, available to select businesses reportedly for four million rials (HK$2,500) a month, say it provides a relatively fast connection to select global websites. An illegal VPN can be bought for as little as US$50.
"You can actually get some work done with this VPN. But it is almost as if you are paying the government to spy on you," said one business user wary that his privacy could be violated.