Seamless levels

So I have completed the first part of my game (thanks to help from some of the users on here!) I’m now looking at the next part which involves pretty much doing exactly what I have done again but slightly differently and more complicated to make the game harder. I don’t really want multiple levels as I want the game to be one continuous game until the player loses. Is it best to use levels to avoid lag? Or does it have no bearing on the game whether you utilise the use of multiple levels or not? I am kind of worried that all these switches and random numbers are at some point going to cause the game to lag? If I was to use multiple levels can it be done so that the player has no idea they’ve just “moved up a level” and so they feel like they’re just playing one continuous level until they lose?