FCP X incredibly slow rendering!

This topic has been covered extensively. There are a number of reasons why FCPX rendering can be slow.

You've stated some of the specs of your computers: cores, RAM, OS, none of which really has that much effect on rendering (except maybe the extra RAM on the desktop — which helps). FCPX has undergone several changes since it's initial offering, in particular, the changeover to directing most of it's processing power to the GPU(s) (with the introduction of "Metal" — a core library for handling graphics). Also, there is a HUGE difference between using the Basic Title and some elaborately animated title perhaps involving 3D text. Without knowing the type(s) of Title...

JPEG and PNG images are not really designed for video editing, they're not a video codec. Every frame must be converted to video. Which codec you have set up for your project could also cause FCPX to move more slowly (usually ProRes 422 or one of its variants are the swiftest). If you're trying to use Uncompressed... ... ... why? (ProRes 4444 is better).

Any spinning platter disk is going to be slower than an SSD (or RAID) particularly if that disk is an external drive. It won't matter much if you have USB3 or Thunderbolt if the drive hardware is the bottleneck for speed.

Those are a few reasons why FCPX can move slowly.

One way for your type of project to get "up to speed" is to layout the images as you want them for the entire project and export just the visual media, then bring it back into FCPX as a video (recommend at least ProRes 422LT project for export). It's called "baking". You'll have to wait out the initial rendering, but you'll save a lot of time afterwards.

Then it's probably that you're running out of room on your hard drive (the one on which the library exists.) OSX needs 10% of the memory available on a hard drive in order to function efficiently. If this is the problem, then deleting all your (unnecessary) render files might (should, actually) make enough room, or consider moving your library to another drive with room.

If... you have plenty of room on your SSD (and it's an external), it might need some maintenance (Why Solid-State Drives Slow Down As You Fill Them Up). How you handle this situation may be dependent on the brand of the SSD. You should also investigate turning no TRIM on your Mac (How to Enable TRIM for Third-Party SSDs on Mac OS X). Some SSDs have firmware embedded that does this sort of operation as well, but if you read the fine print, you'll see you need to leave them on overnight, or something like that, every so often.

HTH

There might be a problem with fonts (though these usually result in crashes rather than slowness).

You say you already deleted preferences, which would be the first thing to try.

Did you try testing with a new library?

There also may be a conflict with other software that you may have installed.

You may want to run the diagnostic tool Etrecheck and post its report here.