Why is my rendering time extremely slow?

Do you mean rendering or exporting?

Larry Jordan says: https://larryjordan.com/articles/fcpx-5-export-tips/ (mostly about exporting)

1. Optimize media, convert to ProRes422 instead of editing raw camera formats. I don't know what the Phantom shoots, it's probably H.264, that stuff takes a long time to unpack. Are you shooting in the 4k mode?

2. Finish rendering before exporting since it is going to get done anyway.

3. Export using ProRes422. If your media has already been optimized, this is quite fast.

4. Export to master file before moving on to Compressor instead of sharing or sending to Compressor.

5. Export to a different drive than the media is stored on maximizes through put by eliminating all of the processes of hunting for media, processing, and writing new media.

We see the topic regularly on various forums so try extensive rummaging around the webs.

Would you by any chance be mixing clips in your project which have different characteristics ?

Be aware that the first clip in your project determine the characteristics for the rest of your project.

I experienced the same problem until I realized that the first clip in my project had been filmed at a 1440 x 1080 resolution, whilst subsequent clips were filmed with a different camera at 1920 x 1080 ; hence FCPX was trying to convert my 1920 x 1080 to 1440 x 1080 to match the first clip.

Cheers

Try a similar thing but with a library in your ssd. My guess is it will be lightning fast. If that is the case the drive connection is the problem. It is the most obvious bottleneck, so that is where you should start.

Your mac is fully capable of working with 4K, etc