It happens to me as well when having larger code blocks with colorful syntax. Rendering code works the way that each syntax color highlight gets its own
<span>. So, the more code blocks and colorful syntax you have, the more stress for your Android's GPU and CPU to handle (rendering is client-side). Since computers are more powerful than mobile devices, we don't really see that issue happen on desktops (maybe on less powerful ones).
There is this issue for HLJS, which suggests the use of async worker, and/or to load big code blocks in chunks. Perhaps Flarum could take advantage of this to improve its performance?