UDN has been updated with a bunch of information relating to effects performance issues.
You can find links to the main VFX Optimization page here...
The documents branch out from the main VFX Optimization page, and I made concerted efforts to link back and forth between topics.
There is a bunch of information there, as well as two new pages for Particle Parameters/Instance Parameters, and the new Collision Actor module which let's you specify per Actor collisions (kismet/matinee spawned effects where you know the actors locations in the scene)
To get into the nitty gritty of diagnosing game thread, render thread, and gpu issues dig into the Core Concepts page. For solutions to issues relating to those threads follow the I am having trouble links.
Core Concepts - diagnose issues
Getting Results - how to fix the issue once it is diagnosed
This information essentially overviews the processes we go through here at Epic when troubleshooting and getting frame rate in line.