We are connecting using MS Graph.  https://graph.microsoft.com  

I don't think there is any caching. 10 seconds isn't too bad compared to most of the systems I have seen.
It will do a new call when you switch folders and back again.
The Graph API is firstly listing the mailFolders (have they got lots?) and then getting the top 200 messages from the inbox. 

When we built our own Graph API integration, we brought it down to about 2 seconds (but we defaulted to the top 50 inbox messages first and then paged though, we only performed a select=sender,subject,toRecipients,receivedDateTime,hasAttachments,importance rather than the cc, bcc and email body as we don't show that in the filing list)

we have a client where its taking up to 30 seconds to load and sometimes brings up the browser timeout warning message asking to wait or cancel the request - is there anything that can be done to speed this up?