It all depends on your situation. If you aren't bound by house rules, then it's completely up to you. If you have several short classes that are conceptually related (even if not logically related) then it might make sense for you to put them in the same text file. The whole concept of maintainability really depends on what makes the most sense to the most people involved. So, if you're in a development house, then it is decided (hopefully) up front what the procedures are so everyone is on the same page.
If it's just you and the cat, then you decide which way to do things so that it makes the most sense to you. The key is CONSISTENCY!!!! Whichever way you decide to do things, then do them that way all of the time, every time. That way, your project will still make sense to you when you come back to it months later.
As far as performance goes, there's no correlation. All of the code files in your project are simply text files. The compiler will gather them all up and combine them into the relevant number of assemblies, so it all goes in the chili, so to speak.