The logo in the footer of the mobile version of the English Wikipedia does not specify dimensions. It is styled to have a height of 1em. To lay it out, the browser has to fetch the image, determine its pixel size, and scale it to match the CSS, which slows down rendering.
It is also served from a volatile URL which includes the branch name. You might want to use "static-current" instead (i.e., https://bits.wikimedia.org/static-current/extensions/MobileFrontend/images/logo-copyright-en.png)