KMail stuck on loading message

I'm pretty new in the world of KDE DE and his PIM (Kmail, Korganizer, 
Kaddressbook).

As far as I used it, I liked Kmail very much for its features and UI but it 
has an annoying bug. It *often* stops working while loading a message. Not 
only new messages, also old ones. It seems to be hanging or waiting for 
something, I see "waiting for message to load" in the message preview pane.

The same thing is happening on serveral distro -- checked on Fedora 27, 
Manjaro and Arch -- and is very, very annoying because it takes a lot of fime 
before it unfreezes and become responsive again. As a workaround, I switch to 
offline mode and then back online, but I wonder if there I'm the only one having 
this problem...

I really enjoy Kmail and so I'd be very disappointed if I had to change MUA 
for this. Just for knowledge, this thing has never happened with other MUAs, 
like Thunderbird or Evolution mail. Maybe something goes wrong between the 
backend (Akonadi) and mail server? Don't know...

Anyway, a few infos about my current setup...
Manjaro Linux 4.9.63
Kmail 5.6.3
KDE 5.11.3
accounts on:
- gmail.com (main account, about 22K emails)
- hotmail.it
- email.it
all IMAPs and SMTPs on SSL/TLS authenticated connections.

-- 
Francesco Porro <[email protected]>