I use the Mailbird desktop client in Windows with about 100 SmarterMail accounts, 5 Gmail accounts and 2 Rogers Yahoo accounts.  Everything was fine until I upgraded from SmarterMail build 8587 to build 8747.  

Since the upgrade, my Mailbird client's CPU usage jumped to an ongoing 85-95%. In Mailbird, there is a "SYNCING" indicator that now switches on and off in all of the SmarterMail account inboxes once every second instead of about every 30 seconds.

In SmarterMail admin, under Settings > Protocols, if I turn off the "Enable IDLE command" setting under IMAP, the Mailbird CPU usage drops to 5% and the ongoing "SYNCING" issue stops.

I suspect this is an issue with SmarterMail, not Mailbird, because this ongoing "SYNCING" behavior does not exist with my 5 Gmail account inboxes, nor my 2 Rogers Yahoo account inboxes.  Only with the SmarterMail account inboxes.  Furthermore, this issue did not exist until I upgraded SmarterMail to build 8747.