How to determine the cause of Thunderbird slowdowns when working with IMAP mailboxes?

Question

Email

There is Yandex traffic rules, there are 10 boxes (all employees work with 3 at the same time), there are 10 workplaces (Windows 10 x64). There is only one problem for everyone - when opening letters, both with attachments and without them, Thunderbird starts to "think" with different strength, and these reflections can in some cases last for 5-10 minutes. Restarting the client almost never solves the problem. Working through the web interface of Yandex itself always gives the desired instant result, but this work option is not suitable for a number of reasons, including. because through the mail client it is elementary more convenient to work with several mailboxes.
What has already been tried :
1) Remove Thunderbird under the root with all profile folders and configuration files for a clean install.
2) Change the maximum number of cached connections (5 by default, decreased to 2 and 1)
3) Rebuild the global database. Helped only with slow search, and then not for long.
4) The client has been added to the firewall and antivirus exceptions. The module for checking incoming mail traffic was also disabled and its settings were changed (the timeout for checking messages was reduced to 10 seconds, mail checking for spam was disabled).
What else could be the problem, where to dig?

Answer 1

It is assumed that there is no virus and the channel is wide enough
1. Close thunder for everyone
2. Run for one and monitor the work. If all is well, start the next one. We monitor again. If let's say it starts to blunt on the 5th client, then
2.1 The number of imap connections from one address is limited by Yandex. Then write to the support
2.2 Check with the provider if there are any imap/smtp restrictions for 10 simultaneous connections (shouldn't, but you never know)
3. Use another mail client for the time of clarification. The same claws-mail, of course it is ugly compared to thunder, but it will do for verification
Well, something like this

Answer 2

try to optimize the connection with Yandex