I agree that there's a lot of worth in having a turn-key solution that's fast, especially if installing dependencies is required anyway (e.g. Ruby isn't necessarily on the user's system, neither is Java, so since you have to install a dependency anyway...).

While it's not like the JVM will never be a source of annoyance for Gitlab admins, speed/user-perceived latency is more than just a feature when porting from an offering that's as polished and good as GitHub. This is especially true for people that would have to rationalize cutting over from GitHub to Gitlab in an enterprise environment.