If the image you are loading is included in the email, it simply does not exist until you open the email. All images and attachments are included in the body of the email as mime encoded text. So until such time as the email needs to display the image, it does not exist, let alone go into some sort of processing or storage.

If the image is held remotely like say on a web server, then the graphic must be downloaded from that server in exactly the same way as a web page is loaded.

Personally I do not see delays, but I use an SSD drive and do not allow an antivirus product to slow down activity on my device any more than Windows defender does. Generally when noticeable delays occur it has to do with the use of the system temp folder to hold files that are created to display or open being held up by scanning or other background processing. This may include having copies of the files uploaded to the cloud, depending on your chosen backup options in Windows.

My system uses a SSD drive also. Everything is scanned by Norton, so that may account for some of the delay. But it is a fast system, with 1GB fiber connectivity. While your answer makes sense, I think they delays are much too long, compared for example to loading a web page with lots of images. I was hoping there was a way to get TB to start downloading in advance - maybe all images.

thx for your help.

Mark

Norton has been on my hate list for most of the time since Perter Norton sold the company to Symantec. In the early 2000S the product turned to a slug and my then teenage son used to refer to our still current license for Norton as "Norton Resource Stealer".

Some 10 years ago they made a concerted effort to actually fix the slow moving high resource usage product they had turned Nortons into. Personally I felt it was to little and too late and have not looked back, other than the rare occasions when I try and install Thunderbird for a friend and they have it. I then spend a long time disabling bit of it so it lets the new account wizard to function without deciding it is a malware attack and shutting down the internet connection to the mail server.

However despite my bias around Norton, have you looked at the system temp folder to see what is there? Windows does a sterling jobs of not managing temp files. I have seen systems with so many hundreds of thousands of orphaned temp files in the temp folder that windows itself is struggling to address them with the available resources. Things get exponentially worse when say Thunderbird is writing more files in there and an antivirus is also trying to monitor the creations and executions. Technically you are limited to around 4Billion files in a folder. The reality historically has been closer to 10,000 before real performance issues kick in.