Activity Stream is timing out when both comments and history entry number for an issue is too high

Summary

When some issue that is about to appear in Activity Stream gadget has a lot of comments (e.g. 200) made after creation of a lot of history entries (e.g. 15000), the loading time can be too high for the gadget to display the results. The timeout for loading gadget data is unmodifiable and is set at 10 seconds.

The problem exists because of current way of calculating issue summary for comment is inefficient. It scales defined as number of comments times number of history entries before given comment was made.

Steps to reproduce

1. Setup Jira with some issues containing 15000 entries in history tab
2. Make 200 comments in the issue.
3. Load Activity Stream gadget

Or you can use prepared backup for Jira 7.2.15: JRASERVER-67865-reproduction-backup.zip
(user: admin / password: admin)
Note: the backup is different than result of steps from above.

Expected result

Activity Stream loads with recent comment

Actual result

Activity Stream fails to load with message:

The activity source for 'JIRA' was slow to respond and isn't included.

Workaround

If you know which issue is causing the problem you can use Activity Stream gadgets' filters

Notes

Activity Stream has safety mechanism that prevents instance from being flooded by Activity Stream calculation. The mechanism will not run the calculation for some time after failure. This backoff time increases with every timeout. For reproduction purposes you can turn off this mechanism by adding JVM property to your Jira: -Dnavlink.failurecache.initialexpiryMs=0.