Atlassian’s Five Secrets of JIRA Performance at Scale webinar shared some useful insights on scaling JIRA performance.
Here is a pictorial summary will be useful for those who missed the video.
1) JIRA 6.4 is 30% faster than JIRA 6.3
2) Custom fields have the most influence on the speed especially on creating issues
3) The number of users does not have much impact on the speed
For details, you can check out the video below or the detailed report at https://confluence.atlassian.com/display/ENTERPRISE/Scaling+JIRA