Not an Apple man,but, it appears you may have a weak machine....especially if trying to edit anything over 1080p. You would need to provide much more info on your rig and which codecs you are trying to edit.
In general, only NVidia GPUs will provide the necessary CUDA acceleration to dramatically improve performance with many operations....AMD is no where as good and provides no CUDA acceleration for "Mercury Playback".
CPU speed needs to as high as can safely be achieved......at least over 3.3 Ghz for starters on all cores, but, best at over 4.3 Ghz overclocked on a "K" series i7 CPU. I'm not sure what you have.
System memory , especially with the new versions of PPro should really be at least 32 GB to take advantage of the now increased use of memory by the program.
Storage speed and configuration is very important to performance. The use of slower, spinning mechanical external drives can slow down and bottleneck your system.....especially if they are full, fragmented, and with " indexing" turned on. Nowadays, many users are using only SSDs in their editing rigs and relegating any spinning HDDs for use as backup drives,or, archiving. For example : your 4TB external drive that holds your video footage may only be running at between 150 to 50 MB per second, compared to an internal SATA III SSD running at over 500 MB per second read or write. Your footage needs to be on a FAST drive for best performance......along with other factors.
Thanks I'm sticking with 1080p right now and its a quad-core 4.0 ghz i7. I transcode my Canon 7D footage to Apple ProRes. I understand that these specs aren't great but what I'm trying to do is the make the best of them right now until I have the money to spend on RAM like you said and different hard drives. I was curious as if my configuration would be a good way to go for now.
....a long time ago, PPro corrected an old problem which occured when trying to edit the native MOV files from the Canon cameras. The MOVs used to trigger the use of the 32 bit Quicktime and this screwed things up because PPro is entirely a 64bit program. So, you SHOULD now be able to edit the native files without transcoding in Premiere. The newer versions of PPro definitely use more memory...I've seen my laptop go over 20GB system memory usage on certain timelines. Increasing yours to 32GB would be good to do. However, without an NVidia GPU, you are in for tough sledding, as without the CUDA acceleration, certain operations can take over TEN TIMES LONGER !!
Another thing to speed up performance : either install one additional INTERNAL SSD for use with active projects. You would place your active footage there along with active project files.....cache and media cache may be placed on boot SSD. The speed of your external drives MUST be slow and must be slowing your performance down. Samsung makes a T1 or T3 USB 3 external SSD which runs almost as fast as an internal drive, ( over 400MB/sec read and write), by using the UASP protocol which is found on most current USB 3 ports. I have one...it works great and is available in 1 or 2 TB versions.