That's the kind of stuff I sometimes hear from people, but about Java instead of HTML5.
Whoever decided to make a game such as RuneScape in HTML5 was an idiot. Seriously, just do a game rewrite in Java and it'll be so much more compatible and probably faster too.
The selection of a programming language for a project has to depend upon your goals and requirements.
Originally, the Gowers wrote Runescape as a Java applet because they wanted the game to run inside a web browser window. They could have picked from any number of other languages otherwise. But they identified the ability to play the game without having to download anything as a major feature. This wasn't a mistake; it worked beautifully. For many years Runescape had a large audience of players who would not have even been able to try playing if a download had been required (i.e. kids in school). At the time the Gowers made this decision, Java was the only option if such a feature was required.
Over time, bits of Runescape have been migrated to C/C++ DLLs for a variety of reasons. I'm sure the developers might have liked to clean things up with a full-scale re-write, but there was no business argument to justify such an expense. But now, for the first time, there is: casual mobile users. This is a huge potential market, and it's worth the expense of a re-write to be able to target it. Nowadays there are a lot more options than Java out there. So which option should Jagex pick? Back to goals and requirements. The entire point of the re-write is to make the game accessible to casual mobile users. Which platform(s) will make that possible?
Apple has gone to great lengths to prevent Java and Flash from running on their iOS devices. This is in large part due to their financial strategy of seeking to get their 30% cut on all apps. Only by breaking or blocking other sources of apps can Apple achieve this. But now Apple has backed themselves into a corner. Their entire official justification for why it was okay to block those other platforms was that HTML5 would be able to meet all of the needs of users. So now Apple can't block HTML5.
So the only platform Jagex will be able to rely upon most of its target users having access to is HTML5. Neither in the original Java case, nor now, did they evaluate many options and pick the one with the best performance. In both cases they chose the only option available to them given the goals of the project. Any performance improvements, while welcome, will be incidental.