Do third-party scripts really slow down ecommerce sites?
Yes - tags, pixels, and widgets are the most common cause of slow storefronts, often consuming more main-thread time than the platform itself.
On most commerce stacks audited in public performance write-ups, third-party JavaScript is the largest single controllable cost. Each tag adds download, parse, and execution time, and tags compete with your own code for the main thread - which is what makes pages feel unresponsive (poor INP).
The fix is governance, not abstinence: inventory every script, assign it an owner, measure its cost, and remove anything nobody can justify. Loading non-critical tags after interaction, or on the server side where the vendor supports it, recovers much of the loss.