Roskomnadzor threatened to block the service completely and said there were more than 3,000 posts containing illegal content on it as of Wednesday.
Twitter, like other U.S. social media, is used widely inside Russia by allies of Kremlin critic Alexei Navalny whose jailing last month prompted nationwide protests.
“The slowing down will be applied on a 100% of mobile devices and on 50% of non-mobile devices,” the regulator said in a statement on its website.
“If (Twitter) continues to ignore the requirements of the law, the enforcement measures will be continued in line with the response regulations (all the way to blocking),” it said.
Twitter did not immediately respond to an emailed request for comment.
Wednesday’s move comes amid mounting efforts by Moscow to exert greater influence over U.S. social media platforms and frustrations over what authorities say is their failure to follow Russian laws.
Last December, parliament’s lower house backed big new fines on platforms that fail to delete banned content and another bill that would allow them to be restricted if they “discriminate” against Russian media.