Symptoms

Atlassian Product can be set into Developer Mode as it is mentioned here: Enabling Developer Mode. This is what it looks like in the setenv.sh (for Linux) or setenv.bat (for Windows):

-Datlassian.dev.mode=true

Diagnosis

This option might be enabled for debugging purposes or trying out an extension or add-ons for JIRA. However, this will force JIRA to re-download the whole page being accessed, instead of caching the data. This could cause a performance issue when the amount of users accessing JIRA at the same time is high, as more load will be given to JIRA to handle. 

Resolution

Look into Setting Properties and Options on Startup page on how to add parameter on the setenv.* file, but this time set the parameter to false:

-Datlassian.dev.mode=false

Or it is also possible to delete the parameter altogether, as this is not required for JIRA day to day basis usage.