For anyone interested, I have finally found a solution to this nagging problem!
In other forums, recently there have been reports of poor performance with Java 8 when it is necessary to access JAR files that are contained in a master JAR file. In taking a look at the CR jar files, I found one such JAR file: XMLConnector.jar, which is a collection of JAR and MAR files for CR classes and various third party stuff.
After unzipping XLConnector.jar into a folder, and adding that folder to the classpath, the slowness is gone, and performance is on par with Java 7, and everything else seems to be working fine with Java 8. So, the key seems to be to not use JAR files that are a collection of other JAR files when using Java 8.
If you happen to be using CR in a NetBeans 8.x platform application with Java 8, I have made a post in the NetBeans forums explaining how to implement the above solution.
NetBeans Forums - How to track down missing resources with NetBeans debugger / profiler