Transferred image size is over 100KB
This means that the URL in question is an image file that is over 100KB, which is generally considered large.
Why is this important?
One of the easiest ways you can improve page load speed is by optimizing images and properly sizing them to the size that is rendered.
That said, the figure of 100KB is relatively arbitrary - there is no hard and fast rule as to what constitutes and image that is 'too large'.
What does the Hint check?
This Hint will trigger for any image URL which is over 100KB in size:
Why is this Hint marked 'Potential Issue'?
This Hint is a 'Potential Issue', which means that it may not necessarily be affecting the site at the moment, but should be investigated as it could be causing issues.
As mentioned above, the definition of 'too large' is kind of wooly in the first place. If you have an image on a page that is doing a particular job, and is necessarily large, then it would be foolish to make it smaller purely to improve page load speed.
Similarly, you may have an image that is 100KB, and it happens to be the only image on a page - then that page will not be so negatively affected by this in terms of load speed.
However, as a general rule of thumb, 'try to keep image sizes down' is worthwhile. Use this Hint to identify your worst offending images, then you can check to see if they have undergone compression and been appropriately scaled, or could be replaced with a different image format to save space (e.g. in the table above, all our worst offenders are gifs, so maybe we should change these to proper video formats).