Is it safe to use Quantserve (Quantcast)?
Some clients use more than one analytics service to learn more about their users. One of those services is Quantserve or Quancast.
The service in itself is safe to use; there is no record of malware being found in their code. Most adblockers will try to block it, and there are some significant privacy implications for your readers; but by installing the code on your site, you're allowing this data to be gathered. The Guardian has a good write-up (from 2012).
However, the implications of loading code from outside your site will always affect performance, since the user will have to download pieces of your page from multiple sources. Quantcast can load dozens of additional scripts.
That's why we always recommend against adding external code on any of your pages and only make exceptions if the benefit is worth it (the most common example, a Facebook share button or a Google font on your theme).
In the case of Quantserve or Quancast, we don't believe the benefit is worth the loss of performance in most situations.