I got the same symptoms, did any of you check your RAM Usage ? Because since the change to the layout of the Chapters, or rather the displaying of them, after a few Chapters the RAM Usage spikes at 4-6 GB at least. Also another thing is that sometimes they just don't load the following pages... hope the team can manage to correct it in a few days or a week :=) but as a leecher i'm just as glad if it is done any time of your convenience.
cheers
EDIT: Dunno if any mod or person with sufficient administrative rights will read this, but here we go.
If it helps any it just happened again as i was reading Seirei Gensouki (https://mangadex.org/manga/21827/seirei-gensouki). Went from Chapter 7 to 8 and went from 700MB to 5621,5MB rather quickly, also it doesn't help any if you just close the tab. I don't have the necessary knowledge to further specify the issue, still hope it helps even if it's just a little
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My setup is as follow: Firefox Quantum Vers. 61.0.2 (64-Bit) - If any information for plugins is needed i can add them.
Settings for the READER:
Only things configured are "Hide Header" and Preloading 15 images/pages instead of the default of 3.