Processed/compressed image resource are not used on client device
e.g. https://typo3.org/article/recap-of-the-combined-sprint-of-typo3org-and-server-team/
Independent of which screen resolution or device simulation is used, the full fallback image is downloaded (Google Chrome Version 76.0.3809.100 on macOS). I'm experienced the same on a native iOS device (iPhone & iPad) connected with 3G.
By looking at the responsive images part, it becomes obvious that no image processing is happening. All source-sets deliver the same original image.
I guess image processing is not configured properly on typo3.org, most probably the settings for a development container are different to the production website which is not reflected in
.envorAdditionalConfiguration.php
<picture>
<source srcset="/fileadmin/t3o_common_storage/news/2019/05/2019-05-22-Combined-Sprint-Stuttgart-Team.jpg" media="(min-width: 1200px)">
<source srcset="/fileadmin/t3o_common_storage/news/2019/05/2019-05-22-Combined-Sprint-Stuttgart-Team.jpg" media="(min-width: 992px)">
<source srcset="/fileadmin/t3o_common_storage/news/2019/05/2019-05-22-Combined-Sprint-Stuttgart-Team.jpg" media="(min-width: 768px)">
<source srcset="/fileadmin/t3o_common_storage/news/2019/05/2019-05-22-Combined-Sprint-Stuttgart-Team.jpg" media="(max-width: 767px)">
<!---Fallback--->
<img class="img-fluid" title="TYPO3.org- and Server-Team" src="/fileadmin/t3o_common_storage/news/2019/05/2019-05-22-Combined-Sprint-Stuttgart-Team.jpg" width="3095" height="2060" alt="" />
</picture>