Does is slow down page loading ? It is already at 6 seconds.
Hmm, 6 seconds isn't good nor is it bad. Where are you getting this number by the way?
So yeah, it just depends really on what you're trying to accomplish. If you optimize delivery by compressing the image and using a CDN service (optional), you should be okay. Make sure the image isn't huge or more than 2000px in size. Remember the larger the image, the longer it takes for the end user to download.
Some kind of paging function would also be good for mobile devices where RAM is frequently at a high premium.
NodeBB causes not-infrequent browser crashes on my iPad2 on longer threads. When I click a link that takes me to the 60th post in a thread or something, there is a long delay as the slower browser begins by loading the op oldest posts in batches. It would be good if there were a way to make it skip doing that and take me directly to the unread posts (without switching the full application to paged mode).
Perhaps on mobile devices you could add a secondary navigation bar with the paging functions (arrows, "displaying X-Y of Z", etc).