Environment

JIRA 7.6.1
Portfolio 2.10.0
Internet Explorer 11
Portfolio plan with 1000+ issues

Steps to Reproduce

Create a project
Import 1000 issues into project.
Create a Portfolio plan and set the project as the source for scoped issues.

Expected Results

Scrolling through issues in the plan would be smooth

Actual Results

Scrolling through issues is delayed, as if the browser is slow to respond to user input. This behavior makes it very difficult to browse issues.

Notes

This was reproduced in a clean 7.6.1 with no custom fields, complex issue filters, or Marketplace add-ons.
On the client, IE11 begins consuming 50-90% CPU when the page loads.
Browsing through the same plan in Firefox, Chrome, Opera, Edge, and Safari do not exhibit the same.
Issue was reproducible in JIRA 7.3.1,7.4.0, 7.4.4 and 7.6.1

Workaround

Enabling the "Faster responsiveness of scope tables" beta feature slightly improves the scroll lag.
Use one of the browsers mentioned previously.