Try profiling to see where the time is being spent. Macro > Slick-C Profiler > Begin Profiling. Then work for a while, reproducing the delays, and then go to Macro > Slick-C Profiler > Save..., and post the result here.
Also, Java, in particular can be a strain on the Preview tool window, because of long JavaDoc comments, and very deep inheritance chains and method overloading, try closing the Preview tool window and see if performance improves.