Actually, I deal with an internal web app that does something similar, poor performance with diffing agent strings being presented, between IE9/11, Chrome, Safari, and Firefox browsers hitting it and getting very very different experiences.
The cause is attempts at code optimizations, some not done well at all. Despite their best efforts, none of our tech teams can blame some grand conspiracy with Microsoft, since no motive exists for this.
But our users find evidence when IE works so much better than, for instance, Chrome. Until a month ago, that is, when the JVMs got to be working properly, and woot, now IE is the slog despite working just as before, and Chrome is blazingly fast. Now it's a grand conspiracy to kill IE use at the enterprise level.
Ya can't win, ya know. whatever you do, if the browsers get different performance results, you're doing it deliberately, because there is some reason...
More reason to avoid web programming. Servicing is still a sweet spot around here.