This really depends on a lot of factors. For instance, how many apps you keep running simultaneously, the theme you use, etc.
How much memory your device has available is what allows for your device to run any particular OS. For example, the Curve 8530 has significantly less memory and therefore cannot support OS6 which uses a ton of memory in comparison to OS5. In turn, the more memory you have, the faster your device. The less memory you have, the slower your device.
I went from an 8530 OS5 to a 9650 OS6 and I noticed a pretty significant difference. The 9650 running OS6 seems to run a lot faster and smoother, however, the more apps I use, the more it begins to slow as the free memory becomes lesser. I've also noticed that with certain themes (such as the stock OS6 themes which I will no longer use), it also causes lag because the theme contains animations and whatnot that are completely unnecessary and only serve to make the screen transitions "appear" smoother when in reality they're not.
Another huge factor is the apps which run in the background. You can view those by holding down the menu button until the Switch App screen pops up. Apps that run constantly will also cause the device to lag as well as cause unnecessary battery drain.
If you're still uncertain, try searching BlackBerry App World for "speed test" or something of that nature as I know there are apps out there which will indicate how well your device is performing.
Hope this helps and good luck!