I'm maybe a little late on this and no Guru, just a little more advanced in getting to know Linux Mint so I hope this helps you along if you have not sorted out your problems yet. Stuff with the RPM extension on the end (keeping it simple I hope) after the full stop are containers that hold all the parts or little programs and scripts needed to build the application or add the functionality to your system. Like if you had a box full of bits from IKEA to make a coffee table. The little programs are the screws, bolts and wood, the scripts are the instructions on where everything goes. When you see RPM it will be described as a 'package'. Linux Mint does not use this package called .RPM, it uses another that can be identified by '.Deb' at the end so you will have the program name maybe the version number and finally the '.deb'.
You have downloaded the RPM package which is for a different brand of Linux (The RPM package type was created for Red Hat linux and is used by some others as well). Linux Mint and Ubuntu which Linux Mint came from use the Debian form of Linux, ergo their packages end in '.deb'.
If you want to download anything always try for a version on the software manager first as it's been tested by lots of people before its allowed there, second choice is synaptic package manager which has the stuff for Ubuntu in it (possibly not tested or not tested as much on Linux Mint). the third option is installing using the GDebi Package installer which installs those '.deb' packages and again opens up the higher possibility of the task of adding your application going less smoothly as their may be slight differences between the main Debian version of Linux and your Linux mint version.
So that's out the way and if I got anything wrong there I hope someone with more experience would add to this.
Now with your printer. If you are using the CUPS web interface have you been to the printer tab then used the pull down administration list that shows you the options there. I would check what that says and see if all the settings are at the optimal for your printer. maybe its only printing in one direction and not on the return or something like that. Check this against your printers capabilities and if its possible. I'd do that before trying to change the driver. If you have no luck definitely hone your search skills on your favourite search engine looking for others with this problem. Don't just search for Linux mint printer speed problems. look for Ubuntu and debian distributions but remember Ubuntu is the most similar to Mint, debian is a little less and if you find info about another version of Linux there maybe a little more difference in how you carry out tasks or where folders you need to access are.
Hope this helps a little