Assuming windows computers:

Try the 3 fingered salute: (ctrl alt delete) all at the same time, then select Task Manager.

In Task Manager, take a look at the Processes tab, the very top row, and check out the utilization percentage of your
CPU, Memory, and Disk. Take a look at the specific row for LR and see what it's using, as well as any other CPU/Memory/Disk hog programs. Clicking on the column (e.g., CPU) will sort the use, just look for the little carrot ^v in the label to note sort order. Same for Memory...

Then use the Performance Tab to check out CPU, then Memory utilization.

Have the Task Manager up and running, then try to use your LR brush that's causing the stall, and take a look at Task Manager to see if it can identify the problem. You're looking for big spikes in something...

The ".7.1" is the point release information desired for confirmation.

Try going into Edit / Preferences.

For "graphics" mentioned above, that may be found on the "Performance" tab. I don't have v5 installed to say for sure if this tab is available in your v5 software. I have mine checked on a v6 install with Win10.

I think the change that will help more is to try the "File Handling" tab. Look for the Camera RAW Cache setting. Assure the "location" is a drive with available space, then increase the Maximum size to something that 'fits' into that available space. I have mine set to 20GB. Consider a number 20GB to 50GB, depending on the available space / drive where you have this folder located.

Restart LR and see if changing the cache helps. If not or only 'maybe', then try the graphics card idea.

I have returned home to take care of other matters. Here is my full response.

First realize that the "Folders" panel in the Library Module is not a directory on your computer rather is where LrC has been let to believe the files are located. And the Explorer/Finder view is where the files actually are located. What needs to be done is to reconcile any differences which result in "Lost" files.

Next I would set up the view of the Folders panel to show the full path to the files. Do the following:
Right click on the + (With a tiny down arrow under it) to find a drop dawn menu. Under "Root Folder Display:" check "Folder And Path" and "Show Photos in Subfolders". Bingo! You will see the full path to the folders.

After you have found the new location of your missing folder in Finder/Explorer Right click on the "missing" Folder in The Folders panel. That will open a dropdown menu where you can select "Update Folder Location...". That in turn will open an Explorer/Finder view where you can navigate to the folder just above the newly found folder and click the box "Select Folder" (or just hit return if you are lazy) and - Drum Role Please - you have reunited the folder with LrC.

I have LR6. The brush does task your computer. I have a Dell Aliieinware ,Intel i7 4 core, 64gb ram & an 8gb GTX 1070 video card gaming computer. The more brushes you use, the more LR will slow down. It slows my computer down. Also when the ac is not on & it is hot & humid out, it gets worse. I seen a vid where Julian Cost used 1 brush as much as she could changing something on the brush functions instead of creating a new brush every time she wanted to do something different. It was an Adobe video on you tube, Not a Julian Kost vid. If I knew the video, I would post the link. Check Adobe's you tube channel & look for Julian Kost , LR.

Good Luck!

The software didn't change or morph into something it was not. It is the constant.

More likely is the computer. Something may be using up resources, memory, disk space, etc. An example might be something like an antivirus program. For example, my daughter in law had a good laptop that wouldn't do anything because she somehow had, with extra caution, two antivirus programs running. Nothing was left for Word!

How much free disk space remains? How much RAM is available? What programs might be running in the background?