A tag manager exists to let marketing teams deploy tracking without engineering involvement. That convenience has a direct performance consequence that accumulates quietly over years.
The manager loads and then loads everything else
The container is a single script on the page, which then fetches and executes whatever tags have been configured inside it.
Each of those tags is separate code from a separate provider, retrieved from a separate server, and each carries its own latency and failure modes.
The page's performance therefore depends on third parties the engineering team never reviewed and may not know are present.
Tags accumulate and are rarely removed
Adding a tag takes minutes and requires no deployment, while removing one requires knowing whether anything still depends on it.
Because nobody is certain, tags from concluded campaigns and abandoned vendors remain in place indefinitely, executing on every page load.
Containers on long-running sites commonly hold a large number of tags of which only a fraction serve any current purpose.
Trigger configuration determines the real cost
A tag set to fire on all pages runs everywhere, including pages where the data it collects is never used.
Restricting triggers to the pages and events that matter reduces execution substantially without changing what is actually measured.
This is the single most effective remediation available, and it requires only configuration rather than removal, which makes it easier to agree internally.
Synchronous behaviour blocks rendering
Modern containers load asynchronously, but individual tags inside them can still write to the document or fetch resources in a blocking manner.
One badly behaved tag can delay the visible page while the browser waits for a response from a server the visitor has no relationship with.
A slow or unreachable third party then becomes an outage on a site that is otherwise functioning perfectly.
Governance is the durable fix
Access to a container is often granted widely and reviewed rarely, which is how untracked tags appear without anyone recalling who added them.
A register recording what each tag does, who requested it and when it should be reviewed turns removal from a risk into a routine task.
Periodic auditing against page performance measurements also makes the trade visible, so decisions about adding a tag are made with the cost in view rather than only the benefit.