We cant be sure for anything. It depends how well they optimize the OS and what features will deliver to the older handsets. If you wait for the official w10, I guess you wont have many problems but on the developer preview things will be different.
I think in general that wp8.1 is smooth in our phones, hopefully MSFT will continue to offer stable updates.
Now from my experience with the Lumia 920, I didn't have many problems, with lags and speed (maybe a few times about 5-10). All the time I am on developer preview, what I mean in when a new preview is out I download it immediately.