I have noticed they have multiple situation, for example when everything's quiet internet is not so bad (despite the fact bandwidth is extremely low for huge amount of people), but when some news came out about government corruption, guess what ? some vpn does not work . In 2009 green movement they closed every https connection.(maybe that was red alert situation)
p.s : https://en.wikipedia.org/wiki/Deep_packet_inspection
p.s. : I use vps from netherlands for bypassing firewall. but It takes huge amount of time and a little money.but the point is 99.999% people don't have this option (I use shadowsocks, sometimes another tunnels) so they use internet the way is or some software like freegate and other but with extremely low speed unbearable lag.
p.s. : pptp, l2ps and others are closed right now. even president rohani couldn't manage the situation . I have heard he did want to do something but supreme leader and his people stopped him.