I too had fx bin lag for a while. Drove me crazy!! It took a lot of searching to find the problem(s). The part of the way solution was to upgrade my video card and also some houskeeping to remove some older or poorly written plugins. I also had snappy response with the fx bin in previous versions of sonar, including X1 and X2. So it is an X3 thing, but that doesn't mean it's a bug. X3 is built differently and I found I needed to stay with plugins that are either better written or more suited to X3. Typically most plugins were fine, but I knew I had way too many old freebies loaded.
Another note: My problems with the fx bin really started becoming noticeable after I loaded the LE version of Cubase that came with some hardware. Even though I uninstalled CubaseLE, the problem stayed.
And finally, last weekend I reformatted my hard drive and reloaded the OS and only the software I needed. It had been over 5 years and I had about 50 gb of crap camping out on my OS. Now, everything including a snappy fx bin is working great. My 5 year old machine is fast, sonar X3e is running great at low latencies. (I have a studiocat daw)
I hope some of this helps you as you try to find the root cause of your observations. It may require some time and effort to undo something that is interfering with X3. My ultimate solution was to have a clean OS. It was needed anyway.
Cheers