Well I know I am not using the most powerfull hardware, but that’s not my point.
Opening a mail in eMClient takes me 2-3 seconds. (Intel Core I5, 4 GB, SSD, Win 10-64).
but with the same laptop, on the web interface of my provider, generally only 1 second (Opera/IceWarp)
IMAP settings
and conversation views are disabled.
I just wonder why?
There will be a difference because it is totally different technology. You can’t really compare a web interface to an IMAP client as they access the server, in different ways.
Saying that, because you have the offline option enabled in eM Client, it should be almost instant. That does not apply to linked content though, and that will always need to be re-downloaded every time you view the message.
What version of eM Client are you using Domi? If it is not the latest version available in the Release History, maybe you could update it and see if there is any difference.
Well, I have just been testing for one week with 7.2.40748.0
( And btw I found all functionalities I wanted so it fits my needs 
)
I am still in Demo mode, I suppose I have to get some kind of licence before I can update?
That’s the latest version 7, so nothing to update. I just wondered because there were some issues with a few earlier version 7s.
There is also the version 8 beta available to download if you want to give that a try. Generally I have found it to be a lot faster. No license necessary.
Just tried 8.0.2703 (687da84)
Same speed, one second for new window, one second for the header, one second for the message.
I would try offline to check the speed, but I can’t find the work offline command in version 8?
Menu > File > Work Offline
You then get a new indicator:
Well, no real difference offline.
it’s still a mystery for me.