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jQuery Migrate is a javascript library that allows you to preserve the compatibility of your jQuery code developed for versions of jQuery older than 1.9. It basically restores deprecated features and behaviors so that older code will still run properly on the current jQuery version and later.
Most websites that use up-to-date themes and plugins don’t require jQuery Migrate on the frontend, and thus the script can be safely removed. By removing it, the browser will have one less JavaScript file to download and execute. As a result, your site will load faster!
Read our article about jQuery Migrate and Website Optimization.