Depends on how much RAM you have. If you have 4gb and are running a virtual machine, OSX will also need to utilize that RAM at the same time. Could slow down performance. If you have 8gb RAM then it shouldn't be an issue assuming you only want to run 1 VM, just give 4gb to the VM and it will fly.
You won't have that problem with Bootcamp because Windows will be running natively and will have sole access to all of your RAM.
The advantage of running a VM is you don't need to dual boot, you can just open Parallels and be up and running. Also you can share files back and forth with Parallels. Also a menu will appear within OSX for the Windows programs, just click on the program and it will launch your VM.
If you have a 2015 Mac Bootcamp will not support Windows 7.
Honestly I've used both and they have both been fine. Bootcamp could be faster because you'll be running Windows natively. VM on the other hand is more convenient if you don't want to dual boot OSX and Windows.