Hi Diethmar, i see the same on my iphone and ipad, new mails take a longer time to be open. 
But i have beta ios (16.3 beta 2) and ipados (16.3 beta 2) on my apple instruments and i means that this is the reason!

i see the mail, but when clicking here the mail will take any time to be shown!

cg

Thank you Claude for your feedback.

But in this case I want to get some clearance around our Windows client ...

Diethmar,

is it the browsing/openening folders in the mail client, or when actions are done like composing new mails.. ?

Could be also some external libraries (like which browser is used for rendering). Does changing the browser in "options" make any difference ?

Just an idea...

It happens many times if I drag and drop items to another folder. In my case QuickViewer is not active. But I do not know if other users use active QuickViewers ...

Can you as we have seen recently check if you have Netbios enabled on windows and see when you disable this if the slowness is there?

I have checked my client environment and Netbios over DHCP has been enabled - -> disabled now. However I am far away from my GW server.

I will observe behavior ...

I haven't upgraded to 18.4.2 yet, but 18.4.1 is definitely slower than 18.3.1, which was also slower than 18.2.1.
In 18.4.1 it's like the whole mailbox refreshes every time you move the curser.
I use the old GW 2014 view, so it's not like there are much data to show.

Can't say I notice any unusual slowness with 18.4, *but* I'm working almost exclusively in caching mode. What I have for a while (basically since 18.X) is some sort of instability when I work with nested attachments. Every once in a while when I open mail attached to other mails, go elsewhee and later come back to the (already open) mail, the client greets me with some sort of blob error and closes. But slow? No.

Caching mode was my religion for many years. But I saw information drifting apart. So there were differences between online and caching. I live in a world of panels. Items will be moved to the right panels, I use categories, follow up flag, ... even my junk mail folder has a higher ranking in my handling of incoming mails.

In my case online and caching show different states - it happens that mails will be not displayed in the right panel, that items will not removed from my todo list. So I cannot trust to get right state of information independent of online or caching 

I work in a simular fashion (cache or online only).

This has allways worked flawless. 

Until 18.x.x (cannot find out wich excact version - but related to the fact that mails belong to a "communication stream" are bounded together. Since then a lot of usues with sending to multiple adress.  
I am not alone in this.