Third-party scripts are added in a single line by someone who is not measured on performance, and removed by nobody.

The invisible costs

• Main-thread time. A tag manager that loads six scripts can add hundreds of milliseconds of blocking time on a mid-range phone.
• A connection to a new origin, with DNS, TLS and often a redirect before a single useful byte.
• Whatever that vendor ships next week. You have granted execute permission on your users’ browsers, permanently and without review.

Practical discipline

Keep an inventory with an owner and a purpose for each entry, and review it twice a year — most lists have entries nobody can explain. Load anything non-essential after interaction, use a facade for embedded video and maps, and set a performance budget for third parties separately from your own code, because otherwise your team optimises its own bundle while the total gets worse.