system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do software installation or updates always go very fast up to 90% complete, then the last 5-10% takes so much longer?
gpt
You shouldn't really listen to anyone here. The answer is that it's implementation dependent. Maybe the engineer wrote it so that there are four steps of 25% each. Copying files is maybe step 1 and is super fast. Step 2 is a big database operation and is slow... etc.
Maybe the steps are totally fake. Maybe there is a simple timer that moves the progress up 10% every second until 99% where the last percent is actually dependent on something. 

Its completely unique to the person who programmed it or a commonly used installation software.