It is entirely possible technically that this could be done. At the ISP you could easily make access to certain sites slower.
For this to happen though the government would either have to run all the ISP services for the country or be able to dictate how they operated. I have no idea whether this particular government wields this sort of power internally.
There could just as easily be a non-sinister explanation. Different websites get there connectivity via different ISPs. It is entirely possible that the traffic for some of the sites is just being routed via a much slower link because of this.
Edit:
Looks like the Tunisian government do censor Internet access.
http://www.indexonline.org/news/20020703_tunisia.shtml
"The government has announced an ambitious plan to provide connections to 800,000 subscribers and the BBC reports that in early 2001, the number of internet users was estimated at around 100,000.
But only two internet service providers can serve private users, both are run by people with close ties to the family of President Ben Ali."
http://encyclopedia.thefreedictionary.com/Internet%20censorship%20in%20Tunisia
"Tunisia has blocked thousands of websites (such as pornography, mail, and translation services) and peer-to-peer and FTP transfer. [Technically, the filtering is made via a transparent proxy and the ports 23, 80, 1080, 3128 and 8080 are blocked.]"