Maybe a poor expression on my part, I think it's the cause of IMAP IDLE. I tested marking an email as read with putting all three mobile, web and desktop client

Maybe a poor expression on my part, I think it's the cause of IMAP IDLE. I tested marking an email as read with putting all three mobile, web and desktop client side by side and change was practically instant in a second. Too bad iOS doesn't support IDLE and goes as low as 15 min only.