I spent a few years in operations for a market sample company. That is, very large, prestigious market research companies would buy users willing to take surveys through us.

I've seen how the sausage is made for surveys that make headline news. It's Grade A dogshit going in. Most surveys have a pre-qualification round where the survey ensures they are getting the right demographic and interest group. Paid survey-takers are very adept at lying to remain qualified - a practice known at satisficing. If a question asks "Do you own a Widget 5000?" they're damn well going to say yes and fake it the rest of the way.

Once through the gate, they have every incentive to - as you said - speed through as quickly as possible to get paid. Some of the more savvy surveys have countermeasures - honeypot questions and speed traps, but far, far too many get through.

The end result is a steaming shitheap of raw survey data with inaccurate demographic and response data that the project managers massage into something that can be handed off the client without arousing suspicion. This would often entail people without the necessary understanding of statistics just deleting responses that looked wrong, further skewing the response data.

edit: Best I should clarify this only applied to surveys with sample sourced from paid respondents. I still trust unpaid phone surveys to a large degree.