The Charts & Graphs for Bitbucket Server app stores all file and commit statistics in Bitbucket Server's database. This information is used to build reports more quickly and avoid repetitive and processor intensive calls to git. The app creates this index after initial installation, and then updates on every repository change.
On large Bitbucket Server instances (more 1000 repositories, 50 - 100k commits per repository) it may require an extra 512 Mb - 1 Gb additional memory to maintain this process. Also, initial indexing may take some time.
Time required for indexing depends on two factors:
Number of files in the repositories
Number of commits in the repositories
The size of repositories and number of branches doesn't make any significant impact on indexing.