system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When loading something, why does the progress bar often speed through the first 99% but stall on the last 1%?
gpt
Often the progress bar is just a representation of the number of tasks or operations that are remaining (to be completed).

The problem is that some tasks/operations may be much more complex and time consuming than others and the progress bar often does not take this into account.

So if there are 100 tasks/operations to complete, the first 99 may be very simple and trivial (like change a setting here, change a variable value here, copy a small text file here, etc.) and the final operation may involve a complex set of steps like extracting a large number of files from a compressed archive or finalizing the installation and validating data integrity of all the files that were installed. 

Sometimes the final step also involves some sort of external process/command that takes a while to respond and the installation program simply has to wait until the external process/command completes and returns a response.