Have you looked that the machine/VM statisics? Or engaged your IT team to look at performance of the machine? I only ask because I also just did an upgrade from 7.5.0 to 8.1.0 last night and our users have been reporting that things are much faster.
It might be that you need more resources allocated to the Jira Server. One other thing you can check is the View System Info in the admin panel and check the "Java VM Memory Statistics" to see how much java heap you have free.
I hope that helps.
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