I have been having loads of problems with pages becoming unresponsive. I have turned off adblocker and that helped on a default Joomla site, however Ark is unusable on another site and eventually I got some big red multithumb error messages, so I have presumed that multithumb is causing the problems (and I use this on lots of sites http://extensions.joomla.org/extension/bk-multithumb) I also have http://extensions.joomla.org/extension/lazy-load-for-joomla on the site too.

Have you tried Ark with multithumb? You can make it work by context and I am wondering if there is a way to use contexts to disable multithumb when ark is in "edit" mode (although this will not show the image at its reduced size).

Multithumb default contexts: com_content.article,mod_articles_category.content,com_tag.tag,mod_custom.content

Fingers crossed this can be solved
Thanks Sarah

I decided to leave it off this current project, I'll try again on another one in the future, thanks :)