Hi,
After upgrading to Apex 5.1. we are facing some delay when a page which includes a collapsible region is loaded.
It seems to be caused by the fact that the following file which is only needed when a collapsible region is included within the page: jquery.ui.button.js
The console gives the following error:

Is it an APEX bug?
Product Build5.1.2.00.09
Universal Theme
Thanks
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Edited title so it shows the root cause.....