That depends what you comparing with. If single SSD on T3 (Thunderbolt 3) will never be as fast as internal.

Apple rate the 2019 internal SSD to be up to 3.2GB/s read speeds.
Fastest single external T3 SSD we've seen are Samsung X5, topped at around 2.6GB/s read speed.

https://9to5mac.com/2018/09/05/review-samsu...able-ssd-video/

Unless you gone to extreme with multiple SSD RAID-0 on T3, you might able to break the internal speed. As T3 can go up to 5GB/s on single channel.

https://blog.macsales.com/37294-tech-101-wh...s-mean-for-you/

https://9to5mac.com/2019/02/25/owc-thunderb...en2-dual-setup/

This post has been edited by wei: Oct 11 2019, 11:43 AM

We don't see the point of partitioning since it is still the one physical drive. Performance of internal drive are proven faster.

But you need to factor in that there are other processes of MacOS/ App that will use the internal drive at the same time FCP are writing or reading to it.

Whereby external drive will be cater solely for one purpose.

It's not a bad thing for external SSD, get two or three and stack-em up you can break the internal speed at no time. You might replace the Mac anytime. The external drive still stays. I doubt anyone would pay so much for a used MacBook Pro with 4TB SSD. If I could afford so I would just buy a new one instead a used one.