The Performance Objectives app search and aggregate data in real time. It does NOT extend client JIRA systems with custom fields and does NOT store data outside your premises. That’s important to be noticed!
That’s why the app performance depends on number of issues that are analyzed in real time to track particular KPI. Less issues – better performance.
The work logs truncation is caused by the JIRA Rest API limitation that Performance Objectives app uses.
The complete work logs might be extracted only if the app sends separate HTTP requests for each issue and in the case of 10 000 issues it will have significant and unacceptable performance impact.
To help users get accurate data for time tracking goals the app detects such cases and provide link reference with impacted JIRA issues. The users then have the option to merge some of the work logs and get the desired view without impacting the performance.
You may check more on the topic with Atlassian official statement here.