It may delay, but I dont know, to be honest. It is an extra risk. Recently, people have been denied entry in the US, and we are talking about visiting. Even parking tickets will be checked. A person who obtained pot, in most countries, obtained it from obscure sources. People related to obscure activities may have sought recreation, and may be super nice people, but the fact is that the databases don't know you and how nice you are.
Most won't admit it, but there 4 types of people:
1 - Those who will never do anything bad,
2 - those who could perhaps do something bad
3 - those who may have made small mistakes in the past and/or are prone to make mistakes
4 - criminals
They will check if you are a good guy. Drug is a factor, isn't it? Processing time is super fast for 1 and 4: Passed and denied right away. For 2 and 3, those checks are longer. It may use a computer and take seconds. Or it may need a login and checks of who you bought stuff from. If the source is someone not dangerous, it may take minutes, but if your source is a crime kingpin you met frequently in a shady nightclub, it may take longer because they don't know your involvement with that person, and they have to check, so there are varied degrees of where pot led to.
Being denied a PR is something different than lengthening the process. For that line, it was said that an application may take longer, not that it will, because, honestly, I don't know. We are trying to look at all the factors that affect time (not rejection), and they have different weights.
I may be wrong, but I believe pot, for creating contact with criminals, may weigh in, because (this I know for sure) even the genuine marriage of a guy who comes to Canada and one year later marries the girlfriend he left in his country takes longer because they ask for proof of the previous relationship.
Pot may be ignored by CIC, but not by CSIS, they will try to know your hidden wrongdoings, not the clear ones. If someone shows up in the database with more lines, it takes time. If it is just pot, then meh, 30 minutes later they send the results saying "nothing more than pot, porn, lots of google search about UFOs, too many hours in COD, cat videos and a speeding ticket was found."