That is indeed a consequence of a poor (high) contention ratio.
To rule out gateway problems, what if you do a speedtest from PTCL to PTCL's own testing server? Do you get similar results?
If not, then I don't think it's the contention ratio at fault but some other back end issue. But if you do get the same results, then you're all getting shafted by their bandwidth chokehold.
You can complain, or switch to fiber (touch wood, I haven't gotten any speed problems but customer service is another story... Prompt, in my case, but in the wrong direction), or switch to someone else.
I'm at the point where I've used Nayatel, but ended up with PTCL fiber cause there was no Nayatel, and now that they're digging up outside my house, I'm contemplating. I get 25Mbps with PTCL (constantly) and Nayatel has 20Mbps (I don't mind) - but the monthly costs will be different considering we also use the landline.
Damnit. Choked competition.