The 300ms tap delay was used to help browsers differentiate between a single tap and a double tap. After a single tap, the browser would wait for 300ms if a second tap were to follow. If it did, the browser would present the viewer with a zoomed version of the content. This artificial delay gave a perception of sluggish mobile browsers compared to their desktop counterparts.
Sites which are mobile optimized no longer need to use this function as they present their content in a readable way to the viewer. With the removal of this delay, it is expected that sites will load faster on mobile Web browsers and there would be no perceivable difference compared to the desktop ones. The double-tap-to-zoom function will no longer be supported. If a user does want to zoom into some content, it can be done by pinching and zooming. 300ms may not seem much, but, the video below gives a good indications of how large the difference can be.
It is to be seen if other Web browsers will follow. Firefox already seems to be working on it. If popular browsers remove this artificial delay, it may force Website developers to ensure that their sites are optimized to be displayed on mobile devices.