CC in and of itself being present in the game will not make your game lag, as far as playing. Tons and tons of CC may make your game take longer to load, but it will still play at the same speed once everything has loaded - depending on however fast your computer is.
Now, some CC CAN in fact cause lag - but that's not just because it's CC. It depends on how it's made. TS4 is very low-poly, and thus easy for most computers to keep up with. If you happen to find a piece of CC, say a bed (something with really fluffy, wrinkly sheets and blankets) that is super high poly - like 50k polys - and you decide you want an 8 bedroom house and use that bed in each room, then your game might start to slow down. However, as I said, that is just because of how that one particular thing is made. If EA made a 50k poly bed, you'd get the same effect. Whether the item is CC or not is not a contributing factor, the game still reads reacts to the file the same way.
As for crashing, that can happen if the content is severely outdated, if it's for the wrong version of the game (I.E a TS2 package file), after a major patch, if the file has become corrupted somehow, or if it's conflicting with something else in your game. Such occasions are generally rare, and can be resolved simply by taking out the piece of content in question. Because of this, it's best to download and install CC in small batches so that you can easily sort through it if needed. Mods (not CC) are the main issue, as they have to be updated nearly after every game update. CC, on the other hand, usually has a pretty long life.