when i open the Buy Mode- Decor- Miscellaneous Decor it takes forever to open and then when i try to place things they move so slow around. Is there a reason this happens. no other box does this the plumbing appliances surfaces nothing just the decor, miscellaneous decor.

It's a normal thing. It's because there are a lot of decour entries and the game has to load them all. The slower your hard drive speed is, the longer it will take.

if i add a hard drive to my computer will that help?

I seriously doubt it would. I suspect there may be plenty of available space still on your hard drive. The thing that makes a computer a gaming computer is mostly the graphics card but you also need processor powerful enough to run it. I see you have a post about collection folders (can't tell if your doing process right from limited info in your post) and S3pe(which I suspect you are trying to merge packages with). The Sims 3 can be a laggy game even with the best setup people have available and there are many posts about this and optimizing the game you can look through. Collection folders, merging packages(either with S3PE or CC magic), optimizing their computer, and getting new hardware are all steps people use(some more expensive than others). I would not recommend buying anything until you have tried other steps to speed up play if you can as that can be expensive.

secondary drive may help overall with efficiency (because in theory you can use two (or more) independent SATA channels or multiple PCI trunked lanes 2x2 or 4x2, depends of the hardware), as long as the system and the data (including swap) are separated. Though, real benefits are not so high in this scenario. SSD will help definitelly (better drives are able to fill the whole SATA III bandwidth, not mentioning PCI ones).

If you have a lot of RAM, try to play with RAMDisk.