It's normal for “Not Responding” to appear in the title bar of a running program when Windows detects that the program isn’t reacting to new requests in a timely manner, often because the program is still working on completing an earlier task. Exactly why depends on the specific program and what was being done at the time and what else is going on in the system as well.

For example, a program might be legitimately engaged with performing lengthy calculations, especially for memory intensive graphics operations or while working with very large files.

Usually the message appears if one tries to do something else or click on or close a program at the time it is still in the middle of another task, where windows has to wait until its done before it can properly restore control of the program to the user for further user input. Interrupting it at that point, perhaps by Ending task could result in losing data or minimally the process already in progress. Usually one just waits until the process is over and it will then restore control automatically. This could take a few minutes or quite a long time depending on your system resources. If it does continue after sufficient time, then it was merely still working.

Note that on rarer occasions, if the program never comes out of that state, there might be some calculation with which it has gotten into a loop that seems to "hang" up the program, and that might require some attention. Or it might be that the calculation simply takes exceedingly longer than anticipated.

In the case of opening an inspection, it's uncompressing all the items included and attached to the report, including images and attached documents. Most items like images can be compressed efficiently when stored separately, like images directly attached in the report writer itself (which only need to load into memory when specifically accessed in the report). But images inserted or merged into document attachments become part of those finally formatted documents and remain uncompressed, thus taking a more memory to store and reopen. Anecdotally reports generally only tend to take a super long time to open if there scores of images embedded in attachment documents. So that might be one factor in your issue.  If so, including more images in the body of your report, and fewer in attached documents or summaries (repeating only the most pertinent ones there) might help.  Also using the option to merge images by means of fields such as cover image, logo, and summary can merge images in at a smaller size than directly inserting images into a document, where a larger original image generally inserts in case of further resizing.

Having a lot of other programs open at the same time can also use resources, or having limited RAM or memory swap space available and other resource factors on a particular computer also could have an impact on background processes taking longer than normal. But essentially "Not Responding" is the computer's way of basically saying "give me a minute while I finish something else, and then I can get back to you"