I have used Thunderbird/Linux with the same isp for many years. A few months ago I started getting long delays before a message downloads and displays. Disabling add-ons has no effect. I get the same delay with Thunderbird on a separate Windows PC. I installed Evolution on Linux and eM Client on Windows. They both display messages immediately. I am running Thunderbird version 60.4.0 (64-bit) and using imap I have two accounts. One is at oz.net and the other is gmail. The gmail account is fine. Only the oz.net account gives me problems. I contacted support for oz.net and they say that they are not getting any complaints from any other users and that the problem is likely with Thunderbird. There do not appear to be any network issues. When fetching messages, CPU usage remains below 50% and Thunderbird never exceeds 9% of CPU. My disk drive is only 15% full. So do not believe I have any system performance issues. I created a log from which I pasted an excerpt below, with my email address redacted. The full log shows that when Thunderbird starts, it gets information about directories and messages for both accounts, which takes only a few seconds. However as seen in the excerpt below, there is an 86 second delay from the time I click on a message until it starts to download. The first line shows the time when Thunderbird finished getting the directory and message data. I had already clicked on the message I wanted to display prior to this. The delay I see in the log agrees with the delay I am getting waiting for messages to display. Note that the directory list completed at 07:13:46, but the message download did not begin until 07:15:02 I have already tried many debugging suggestions I have found online, but none seem to help. Here is the excerpt: 2019-01-27 07:13:46.107992 UTC - [16423:Unnamed thread 0x7f6154a14700]: I/IMAP 0x7f61545b0800:mail.oz.net:A:CreateNewLineFromSocket: 4 OK LIST completed 2019-01-27 07:14:20.954395 UTC - [16423:Main Thread]: D/IMAP proposed url = INBOX folder for connection INBOX has To Wait = FALSE can run = TRUE 2019-01-27 07:14:20.954617 UTC - [16423:Unnamed thread 0x7f6160716a60]: I/IMAP 0x7f6160405000:mail.oz.net:S-INBOX:SendData: DONE 2019-01-27 07:14:21.443127 UTC - [16423:Unnamed thread 0x7f6160716a60]: D/IMAP ReadNextLine [stream=0x7f615c5dbc00 nb=21 needmore=0] 2019-01-27 07:14:21.443145 UTC - [16423:Unnamed thread 0x7f6160716a60]: I/IMAP 0x7f6160405000:mail.oz.net:S-INBOX:CreateNewLineFromSocket: 11 OK idle complete 2019-01-27 07:14:21.443188 UTC - [16423:Unnamed thread 0x7f6160716a60]: I/IMAP 0x7f6160405000:mail.oz.net:S-INBOX:ProcessCurrentURL: entering 2019-01-27 07:14:21.443318 UTC - [16423:Unnamed thread 0x7f6160716a60]: I/IMAP 0x7f6160405000:mail.oz.net:S-INBOX:ProcessCurrentURL:imap://{my email address}:143/fetch%3EUID%3E/INBOX%3E88650: = currentUrl 2019-01-27 07:14:21.452617 UTC - [16423:Unnamed thread 0x7f6160716a60]: D/IMAP SHELL: URL imap://{my email address}:143/fetch%3EUID%3E/INBOX%3E88650, OKToFetchByParts 0, allowedToBreakApart 1, ShouldFetchAllParts 0 2019-01-27 07:14:21.452777 UTC - [16423:Unnamed thread 0x7f6160716a60]: D/IMAP FetchTryChunking: curFetchSize 20052 2019-01-27 07:14:21.452791 UTC - [16423:Unnamed thread 0x7f6160716a60]: D/IMAP FetchMessage everything: curFetchSize 20052 numBytes 0 2019-01-27 07:14:21.452809 UTC - [16423:Unnamed thread 0x7f6160716a60]: I/IMAP 0x7f6160405000:mail.oz.net:S-INBOX:SendData: 12 UID fetch 88650 (UID RFC822.SIZE BODY[]) 2019-01-27 07:15:02.271749 UTC - [16423:Unnamed thread 0x7f6160716a60]: D/IMAP ReadNextLine [stream=0x7f615c5dbc00 nb=72 needmore=0] 2019-01-27 07:15:02.271757 UTC - [16423:Unnamed thread 0x7f6160716a60]: I/IMAP 0x7f6160405000:mail.oz.net:S-INBOX:CreateNewLineFromSocket: * 2023 FETCH (FLAGS (\SEEN) UID 88650 RFC822.SIZE 20052 BODY[] {20052} 2019-01-27 07:15:02.271774 UTC - [16423:Unnamed thread 0x7f6160716a60]: I/IMAP 0x7f6160405000:mail.oz.net:S-INBOX:STREAM:OPEN Size: 20052: Begin Message Download Stream