Slow Outlook 365 when replying to a long threaded Email

By | June 3, 2021

Did you notice that when you use the web version of Outlook 365 when you reply to a long-threaded email, every time you type it gets slower and slower. So slow to the point your computer crashes and gives up? Well you are not the only one, there are many companies all over the world that are complaining, and it has been brought to Microsoft’s attention already and looks like they are finding a solution on how to fix this major issue. If you can’t wait for that fix, well I have a small fix that will make your lives easier so continue on to this article as see how easy this is to overcome.

We had the same issue in our company, we just migrated from Google to M365 and people are frustrated, it’s a big change so non-technical people will have a change fatigue and have to undergo learning a new tool then issues like this arise, chaos comes next. So why is this happening at the first place. If you look at the console when you open an email in Outlook, once you start to reply on an email with a really long thread you will notice a lot of traffic is being generated. Item number 1 on the photo below, you will see 65 requests after typing spacebar which triggers the event to perform a GET request item number 2. Now why is it doing that? This is basically Microsoft’s Proofing tool to check if an image is valid or not.

That’s a good functionality to make sure what every you put in there has a valid image but if you are in a long threaded email with 22 embedded images every time you press spacebar you will have 22 requests to the server, so if you are typing a letter with 100 space bars that 2200 requests made, crazy right. Now add your own signature with one or two embedded photos, forward it to your colleague then you colleague will have a worse result, as the requests are now up to 24 rather than 22.

Now there are many ways to tackle this, first if you make your messages show as “grouped by conversation” then you won’t have this issue but some people are used to the old ways and don’t embrace change, perhaps they are so used to it, they don’t want to let go of the individual messages style. If this is your case, then move on to the next steps.

Now let’s check who is the culprit and let’s neutralize it, blocking the traffic that goes out, so it does not request multiple times.

If you check at the codes and search for “” you will see there are 22 images on this email that is being refreshed or requested every spacebar (item number 1), now to eliminate that we need to find a common attribute with this HTML tags. If you will see in item number 2 they all have the attributes data-imagetype that is set to AttachmentByCid. This is the one we are targeting, basically remove this so it does not generate extra traffic.

First let’s select them all by opening your console and selecting the item with the same attribute values.



on your console then press enter, you will see the 22 elements with the same attribute

Now you had selected the right items, all you need is to remove them by adding remove at the end


Select it again, it should give you a 0 count.

Now go back to your email, reply to it again, it should not generate any additional traffic anymore.

Now if you want to roll this out to your users then the easiest way is not to train them how to use Chrome Console but create a bookmark so when they experience this they can just click that bookmark which will alleviate the issue. Just take note this will remove all embedded images.

To do that is just create a bookmark. Type a name you like then in the URL type this


Now your users just need to click this to make the replying experience faster.


One thought on “Slow Outlook 365 when replying to a long threaded Email

  1. Mike McNally

    This is fascinating (and depressing). However, I’m experiencing slow typing in a traditional email “reply reply reply” situation, and there are no embedded images at all. When I start typing a reply, I see an inexplicably large number of POST requests, but character by character as I type I see nothing. However, it is noticeably slow. I suspect that in addition to the image checking problem you documented here, there is some sort of client-side inefficiency (to say the least) that happens on every keystroke.


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