Why is deletion speed in Windows often periodic?

There are a few reasons:

1. Data Continuity: NTFS is implemented where it writes sequentially across a disc, so if you edit a file - NTFS may have to store the changes in a non-contiguous sector on your hard disc. That means that to delete a file, it may need to jump from sector 2 to sector 100,000 (hypothetically, of course) and then back to finish the file. Then it finds the next sector of the next file to delete and again could repeat that process.

2. Data types: Say you are deleting a folder in temp, it may contain large files and small files. Some of those will be quick to process and others may be slower (whether that is size or complexity of data - i.e. how many sectors need to be processed to finish an "item")

3. @Ramhound mentioned the "items per second" piece, which appears to be confirmed by your deletion dialog (Speed: 1.082 items/s). So deleting a huge file, a non-contiguous file, or a small file all plays a role in the number of items per second.

4. CPU Prioritization: This stuff is done in clock cycles, so it is likely that your CPU may be running other processes during the cycle and that may affect the speed (this is conjecture on my part)