Ciao,
I have a doubt and maybe someone can help me to clear up.
Currently i have all my photos stored on an external usb drive (a wd element connected to my laptop via usb 3 ).
My laptop is a Dell Xps 9570ā¦
My doubt is:
where would you hold the photos? it would be better to keep them on the internal pc ssd? could performance improve compared to processing them directly from the external hard disk?
Thanks in advance and sorry for my english! 
usually, it is best to keep things internal. My experience is, that wd element drives are on the slow side of things too and your internal ssd should outpace any hdd anyway.
You can find the difference by a little test: Copy a folder of images onto your internal ssd, work the images and see what you get in comparison. Things should feel snappier.
I put the photos that Iām working on (or will be working on in the future) on my internal SSD. Completed photos and descriptions of them (plus originals, .dop files, other intermediate files in a separate folder) go to a slower and larger storage drive after all the work is done. I also back up everything using Backblaze and external HDDs.
Sounds good too!
USB 3 external SSD drives are just as fast on Mac for processing as internal SSD. The downside is that on silver towers, the external USB 3 drives force eject on sleep. I do both (some of my files are on external USB 3 SSD and some are on internal SSD).
4K RAW or pro res video editing is where you really need to be using the internal SSD on a fast bus.