Git can become slow if a repository exceeds certain thresholds (read this for details). Use the scripts explained below to identify possible culprits in a repository. The scripts have been tested on macOS but they should run on Linux as is.
Hint: The scripts can run for a long time and output a lot lines. Pipe their output to a file (./script > myfile) for further processing.
Use the git-find-large-files script to identity large files in your Git repository that you could move to Git LFS (e.g. using git-lfs-migrate).
Use the git-find-lfs-extensions script to identify certain file types that you could move to Git LFS.
Use the git-find-dirs-many-files and git-find-dirs-unwanted scripts to identify directories with a large number of files. These might indicate 3rd party components that could be extracted.
Use the git-find-dirs-deleted-files to identify directories that have been deleted and used to contain a lot of files. If you purge all files under these directories from your history then you might be able significantly reduce the overall size of your repository.