According to [a Firefox test](
https://www.webpagetest.org/video/compare.php?tests=170827_QE_b683c0c2f14bc5ff574f0b42eb53ba5b%2C170827_RD_01e5db455f7c5041cbbc7335c656a333%2C170827_RA_15b2582f567ef4a86732d9a371a566ae&thumbSize=100&ival=100&end=visual) and [a Chromium test](https://www.webpagetest.org/video/compare.php?tests=170827_ER_34f2816d9c794a7807a70e197ea3cd4b%2C170827_Z9_9a947c6c1ce845681b1295e678156ef6%2C170827_9Q_a7bcdb12413c1f22124c8f7ea17bcee9&thumbSize=100&ival=100&end=visual), the [USA banners being currently tested](https://commons.wikimedia.org/wiki/Category:Fundraising_2017) delay page loading by about 1 second in Chromium and 2 seconds in Firefox.
Some users have reported that in this timespan their browser is frozen, and I had a similar experience (Firefox 55.0 and Chromium 59.0.3071.104).
Those banners tamper with the content in various ways; I guess the JavaScript they use has room for improvement. Their definition can be found somewhere among https://meta.wikimedia.org/w/index.php?title=Special:CentralNoticeBanners&offset=0&limit=100&filter=B1718_0823_en6C