What can I do to improve the performance?

There are easy do-it-yourself upgrades that will give you a speed boost.

• Install more RAM. A Mid 2012 13" MBP can use up to 16GB total. You generally get better performance if you install RAM in matched pairs (2x4GB, 2x8GB).
• Install faster storage. In order of increasing speed, your options for replacing your stock 5400 RPM hard drive are:
  • 7200 RPM hard drive (examples: Seagate Momentus XT, WDC Black)
  • SSHD (platter HD with large solid-state cache)
  • SSD
  • the price-per-GB of these drives increases with the speed; traditional hard drives (250GB-750GB in 7200 RPM) are cheapest, hybrid drives (128GB-1.5TB) are more expensive, solid-state drives (128GB-1TB) are most expensive.

The installation guides are available from iFixit, as are several of the replacement parts; the parts are also available from a variety of online retailers.

Graphics applications benefit from more RAM, since they allow you to open more/larger images without swapping out to comparatively slower virtual memory. But Photoshop in particular benefits from faster storage, since it uses its own virtual memory scheme separate from the one used by OS X. If the disk where the virtual memory space lives is much faster (as with an SSHD or SSD), then you take less of a speed hit when the program starts moving data back and forth between RAM and disk.