V5’s frame rate management was inaccurate… what you’re seeing in V6 is a more accurate representation of your system and it’s abiltiy to “keep up”. If V5 was accurate, you would have been seeing the same thing. The fact that you’re not is the result of what I would call a bug. If you do not want the degradation to occur, then set the FPS setting to something like 0.001.
Now, it’s possible that frame rates are actually slower than V5’s… in which case we need to figure out why. But don’t judge frame rates by whether or not bounding box display kicks in… Run some tests using Testmaxspeed. Start with something simple, like an empty scene…and work up from there…but make sure all settings are exactly the same so it’s an apples-to-apples comparison.