Iran is tightening control of the internet ahead of next month's presidential election according to users and internet experts in the country.
Social network users previously sparked violent street protests over claims of election fraud after voting in 2009.
The authorities deny such claims, but have not explained exactly why service has become slower.
Businesses, banks and state organisations have been suffering widespread disruption to the internet, local media reports.
"The internet is in a coma," said the Ghanoon daily newspaper in a report published earlier this month.
Facebook, Twitter, YouTube and numerous other sites have been censored in Iran since massive street demonstrations followed the re-election of President Mahmoud Ahmadinejad in 2009.
Those protests, instigated online, were stifled by a heavy-handed crackdown that led to numerous arrests and some deaths.
Observers now say the authorities are choking the internet to prevent a recurrence of the protests.
'VPN practically inaccessible'
A network supervisor at a major internet service provider in Tehran, who spoke under condition of anonymity, said his company had been unable to address complaints about slower speeds, particularly accessing pages using the HTTPS secure communications protocol.
The problem also affects what websites people can access.
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, which is illegal in Iran, 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.
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, but he did not elaborate.
Authorities refuse to officially confirm the new restraints, but former officials and media reports have accused the Supreme Council of Cyberspace of ordering them.