I've just noticed that organisations that have a large amount of historic email activity are SLOW to load and render. You're lazyloading the content ... but when a new set of emails is loaded in, I think it's loading the entire email in a hidden expand/collapse accordion - rather than just the visible summary.
Can I suggest that you only load the visible summary when the email history is loaded on the organisation page? And then when the expand/collapse is hit, you then load the email body?
You could take it further - perhaps this could be done selectively - so on organisations that have < 50 email history items, you load the lot, and >=50 you load the visible summary only and trigger load on click expand. For info, the one I'm on has over 500 emails.