You can take a look at some ongoing performance-related issues at https://gitlab.com/gitlab-org/gitlab-ce/issues?scope=all&utf... as well as some already completed improvements at https://gitlab.com/gitlab-org/gitlab-ce/issues?scope=all&utf...
There's also a plethora of ongoing efforts on making our hosted instance GitLab.com even faster. Lots of issues to look over at https://gitlab.com/gitlab-com/infrastructure/issues?scope=al...