Unless they are purely talking about websites, then I don't see why would they even bring app native/hybrid apps...
I've profiled a lot of web pages/apps and this hasn't usually been true in my experience. Occasionally JavaScript is the culprit but that is more often to be due to really bad O(n-squared) algorithms and script that forces layout. In contrast, the rendering engine combined with complex CSS is often the cause of "janky" page behavior. Do you have some profiler runs that show a JavaScript bottleneck on a webview that doesn't exist when using Nitro or some other browser?
Ok, it may not be ENTIRELY the JS engine's fault, but it's definitely the biggest part of the problem...