Editing off HDD causing playback slowdown?

A lot of the 4k DSL cameras from 4 years ago using H.264 listed the speed as 100mbps. That is 100 megabits per second not Megabytes. 

The Red One and BMD Pocket cameras can have speeds of 250 and even 350 MBPS. That is Megabytes per second not megabits.

If you are edting 4K H.264 a single mechanical hard drive will work.

If you are editing 4K or 6K Pro Res or R3D files a single HD might not allow for mutliple layers.

It is not uncommon for the Red Cameras and the BMD cameras to use 2 tarrabytes or more of storage if you a recording video all day long. You ned a lot of storage space and good sustained data transfer rates for those cameras. A RAID system like below might be a good option depending on your needs. You can check out the benchmarks.

https://youtu.be/hfT9ivfbJ98

Neil covered quite a bit so I'm just supplementing. I have a similar computer build to you, Zoom. I have an NVME for OS and Software, an NVME for media cache and previews, and the weakest link of my setting is that I primarily edit off of external HDD drives. I/O speed does become an issue in certain situations, but I tend to work with low bitrate proxies so the external drives still tend to work okay. (That being said, I am looking into a RAID DAS to upgrade my work-from-home situation).

It sounds like you don't really have too much to worry about in terms of I/O speeds, but it depends on the bitrate of your media and if you're working with multiple streams. If you're doing multicam, that's a lot more data that has to go through. You can keep an eye on your hardware utilization as you work. Some media codecs like h264/5 are a lot more difficult to work with, and of course the addition of effects and color grading will further add to the processing required to decode.

People often forget the cache files, which are a large part of performance in Premiere. So add where you put those into the equation.

An internal spinning disc is typically mostly ok for 1080 playback if the only thing that Premiere is accessing on that disc is the media clips.

I have an SSD for the OS, then an Nvme SSD for the cache files, and then SSDs for my projects and media. I use a few large spinners for general storage. It still seems to work best with a 6-core CPU and 32GB of RAM, 1060 GPU.

I've got a new 24-core Threadripper with SSD for OS, an Nvme SSD for cache, and will be loading in my other drives from this current machine. Even with that beast, I'll be keeping OS/programs, cache, projects and media all on separate discs.

Now ... if you've got one of those 10Gig-e RAID arrays, well ... then you've got rather high-speed storage that can probably hold numberous parts of the process.

Neil