I have identified two issues related with this behaviour with textarea fields and codemirror

Too many String Parameters
In a job with more than 30 String parameters, each parameter include a textarea field with HTML preview. As workaround, you can set the "Markup formatter" to "Plain text" in "Configure global security".

textarea resized (shell script,...)
the response times are too long when trying to configure a job with resized textarea fields.
It seems there is a javascript function that resize textarea fields to be big enough to see the code in it, this function seems like have poor performance, so when you have a lot of textarea fields, it impacts on render page time.

Additional info:
Maybe the bug was introduced in https://issues.jenkins-ci.org/browse/JENKINS-25455 over this change https://github.com/jenkinsci/jenkins/commit/e9aeaf11b7171aed80381e8f66c5fb7056960141

and seems these concrete lines in textarea.js are involved in the bug:

         //refresh CM when there are some layout updates
         function refreshCM() {
             codemirror.refresh();
         }
         layoutUpdateCallback.add(refreshCM);