You can't imagine how many times I've heard people complaining that Jira Server/Data Center is extremely slow. As an Atlassian expert, I'd like to say that it's not true. I mean, it could be slow as any other app, but when Jira is treated with care by professional admins there is nothing to worry about. Believe me, I can prove it with many examples. But yes, you have to pay some attention to you data base and virtual machine configuration if you use Data Center. If it's not something you've been dreaming about, try Atlassian Cloud and stop to worry about performance issues at all.