How does git reset complete so quickly?

Question

I have a chromium repository which is around 60GB in size(30GB downloaded from GitHub). I made quite some changes to the point I messed it up. Permanently deleted files of around 2GB in size and added almost the same amount of files in the repo from another source. After the build failed multiple times, I did a git reset and it took around 15 seconds to bring the repo back to its original form. So how does this work? Does the system keeps track of deleted files locally as my internet connection is really poor and downloading 2GB of data usually takes 1 hour so downloading from the internet is not possible.