The other thing is, while too few visleafs (or the wrong ones) can slow down fps in some circumstances, too MANY visleafs DEFINITELY will, and will also bring your complies to a screeching standstill if you aren't careful.
This is really a situation where a picture is worth a thousand words. Load your map, hit f9 to compile, and JUST select bsp, not vis, not rad, and don't run tf2. Once you've done that go to Maps -> Load portalfile. This will let you see exactly where your portals are.
Sometimes you'll find that a wall or an object is causing the blue lines to project a long way from the actual object, and you have small visleafs being created halfway across the map from the object creating it. That's the situation where you would use a hint brush to chop those blue lines off at the source.
On the other hand, there's a situation in the tutorial where you would use a hint brush on a corner to make MORE visleafs to reduce rendering load in-game.
One final thing. Even before hint brushes and area_portals, get familiar with func_details. When you have a brush item, like a column, that really shouldn't be creating whole new areas for visibility calculations, making it a func detail is essential. There's more information on that in the tutorial too.
But this will all make a heapin helpin more sense once you load a portal file for the first time.