This problem with VisualEditor's performance was reported at MediaWiki.org by User:Leaderboard, who was editing on the English Wikibooks. The problem is intermittent or at least variable in severity. It may depend on the size of the page. It appears to happen at MediaWiki (testing in the sandbox) as well, after about a page's worth of text has been added. Here's the problem in his own words:
First , it is too slow to display text which I am typing. It shows it in a slow and slideshow speed. Looks like it's quite slow. And , on Task Manager , it shows that it is using one and only 1 full(utilizes 100% CPU for that) thread for this. It stays at full utilization when I'm typing and becomes very slow. I'm using Internet Explorer 11.
When testing using Performance Dashboard , it stays at 5-15% during idle and while typing , it spikes to 50-55% of total CPU usage.
Running on a Dell Inspiron 6400 with Intel Core2Duo T7200 @ 2Ghz and 4GB DDR2 with Windows 8.1 tested."