This would have to be a SATA III 2.5' SSD drive like the Samsung 850 EVO, this would be much faster than the current hard drive but still a lot slower than the PCI SSD in the latest MacBook Pro. An option would be to replace this with an SSD drive. However I suspect you still have the original hard drive fitted.
As you have 16GB you already have the maximum amount of RAM it supports. On top of that it is only an i5 CPU rather than an i7. Your MacBook Pro being a 2012 model is now five years old and therefore considerably slower than current models. DaVinci, Final Cut Pro X, and some will not. Some software will take advantage of this e.g.
As others have discussed it is possible to connect an external box via Thunderbolt into which you can fit a standard PCIe vide card. The GPU chip is part of the logic board so cannot be upgraded.
