Boosting the stats.
Boosting the carryweight and dragging along tons of weps and ammo from many different mods and DLCs.
Stuffing a container with dozens of guns and tens of thousands of ammo rounds (for example, moving the entire content one of the of 20th century weapons mod's TEST containers location - Alexandria Arms, Citadel... - into a single locker in the Player's Megaton House).
Adding perks without having what it takes to have them.
Using a companion mod to move NPCs and have them stay put forever at one place (for example, having ALL the vendors in the game) waiting near the Player's Megaton House.

I don't know the answer to your question, but did want to chime in with a quick warning (this is true for the xbox version, don't know about others). If you leave a weapon and ammo for that weapon both in storage close enough together there is the possibility that hostile NPCs could go and take the weapon. They won't do it if there is no ammo for it, so either store you ammo some place far removed from the weapons they go with, or carry all your ammo with you all the time. There have been reports of people placing weapons and ammo in a container in their Megaton house then finding them gone the next time they came back, eventually finding them in someone's inventory. They don't even have to be hostile to you, if a fight breaks out for any reason NPCs will grab the best weapon they can find.

Do you mean slowing down the game as in making it laggy or stuttery? The only one of those things that would cause that is moving all of the vendors to Megaton could cause Megaton to be laggy for you if your machine can't handle it.
If you mean breaking the game as in making it too easy, well it's your game and only you can answer that question.