Does Google Tag Manager slow down the site?

Certainly No. The real fact that Google Tag Manager is helping to speed up your pages loading, asynchronous loading using a single trigger. I can give you a few real-time examples of:

  1. GTM replaces old manual code (JavaScript, JQuery) written on every page earlier for tracking purposes.
  2. Another example, you have written code (link) for download a single PDF file, but GTM single trigger to download all PDFs.
  3. GTM all tags are loading asynchronously, meaning that when it executes, it does not block other elements from rendering on the page. It also causes the other tags that are deployed via Google Tag Manager to be deployed asynchronously, meaning that a slow loading tag won’t block other tracking tags.

In earlier days, If you would like to add marketing tags then you should ask developers to help to add and deploy it, but using GTM any marketing persons like SEO people can add tags, triggers via GTM, and start to track. Now many people are started moving some tags on GTM custom  HTML block instead of hard code in web pages.

So, as per my point of view, GTM really speeds up the site loads. If you have any suggestions, questions, please feel free to comment below.