How much faster is DR with a faster SSD - cacheing/rendering

Before I invest 1k$ for an eGPU I wonder how much DR would gain if I just invest in a faster SSD for cacheing.

Actually I use a iMac 3.5GHz quadCPU, 48GB Ram, Radeon 575 with 4GB Ram.
I have the comlete OSX and everything on an external SanDisk extreme SSD using USBC-3.2 and that way I have around 900MB r/w speeds.

When I create optimized media for 4k projects it takes quite a while, and also rendering a 20min 4k video is taking around 15-20minutes.

I am just curious, what if I invest like 300$ and get me a X5 SSD which will more than double my r&w speeds. Has anyone made a test on how DR benefits from higher SSD speeds? when using todays top SSD like the X5 compared to just USB3 SSDs?

It depends on cache format. For uncompressed you need fast disks as those will be big specially for UHD+ resolutions. For ProRes etc. cache single SSD should be easily enough as 60p UHD ProResHQ is 220MB/sec. If you have single HDD it will start bottlenecking at those speeds.

Regardless of disks if you have poor CPU/GPU then any faster storage won’t help. You want balanced system. Watch CPU/GPU load during cache. If they are not used quite heavily your disk may be slowing things down.
Forget about simple assumption that adding SSD will speed your caching few times. It can, but it may do nothing at all. It all needs to be observed and calculated.

In your case if current disk does 900MB/sec and you use default ProResHQ format new SSD won’t do anything. It’s just your setup. Watch CPU load. If it’s passed 50% then it simply can’t do it faster. For me it sounds like speed is about right for your setup and problem is in CPU/GPU not disk.

You can try generating optimised media at half resolution (it will be much faster). It's in project settings (Master Settings). You just force it to use half resolution. Of course then your preview will be just HD, but it should be fine. Just when you do final export never tick "use optimised media" as you want final master to be done at full resolution from original file.