it looks like people are getting a bit confused here... firstly, single user mode is not the same as safe mode (which is what machead3 is referring to), secondly, the 13" macbook pro doesn't have 2 graphics chips like the 15" model does, so you cant swap between them, although there are energy saver modes that effect the battery life of the machine... none of those functions will cause the machine to lag or slow down though.
Firstly, are you comfortable opening up your machine, or would you rather have someone else look at it for you? i can give you suggestions to track the fault down, but dont want to suggest anything that you wouldn't be comfortable doing yourself.
Any internal connection can cause the machine to lag that communicates directly to the logic board. things such as the HD (but you would usually either hear abnormal noises coming from the drive, or the beach ball instead of lagging, but its still possible), it could be your HD flex cable (which you can eliminate by booting from an external drive via USB or FireWire), or it could be anything else, such as your iSight camera, airport/bluetooth card, ram module, ram slot, etc.
The thing to do would be to eliminate the faulty component. you can start by booting up in safe mode by holding down the left shift key when you start the machine. this will boot it in a simplified version of the OS with most of your hardware functions disabled... then i would open it up and disconnect everything that is non-essential to the machines booting needs to ensure you the fault isn't with the main logic board or the ram slots, then re-connect one device at a time until the fault re-occurs. I have seen an iSight controller board cause the fault you are seeing on an 2011 range. although it is seen as a USB interface when connected, it passes both data and power, so can cause interference if it is defective.
i hope this helps