Do you have a SSD (solid state drive) or a hard disk drive (traditional hard drive)? If you don't have an SSD, that's what you need. That will solve your speed issues without the need for additional RAM.
EDIT: just post your specs, please. There's really no way to get a decent answer until we know what hardware you have.
Yeah a 500 g ssd will be a huge improvement - mid 2012 is easy to upgrade and still a great lap top with a faster drive - 4 g memory is on the low side - you might look at bumping that up too - but the drive is where the big improvement will be.