Actually I don't particularly mind the onemanga scans on /a/, because they load in a split second (unless zip is having a bad day). It lets me see the frame/page in question nice and fast.
If it's a chapter I have already read, then I already have it on my comp in high quality. If it's a new chapter, I am going to download that chapter in high quality.
It's not like these threads are expressly for delivering people on /a/ a full chapter of the manga in shitty onemanga quality - it's just a page for the purpose of starting a thread, or conveying a certain entertaining moment in the manga.