[Afonso Quelhas]

High ping isn't what causes this. I've seen people with 200 ping driving around just fine. Their car will appear to slightly speed up and slow down a bit while drafting and you want to leave them a little extra room just to be safe but otherwise it's fine.

Unstable ping is the problem (Ping that jumps around a lot between 100-200 for example)

That is true. And I'm going to take advantage that you are talking about this, to report some weird behavior from some "lagging" drivers lately.... It's been maybe 2 months now, that sometimes I get some drivers that in a very weird way only start lagging when you start catching them on track and amazingly stop lagging just after you have passed them....
Always thought it was a coincidence .. But it has happened more times since then. 
Next time I will officially report it because I believe that it is intentional.