I am planning to get an external storage for Ventoy use. My current random usb 3.1 stick writes .iso-files with only 10-15 mbps only. I found this webpage for usb stick speed comparison: https://ssd-tester.de/usb_stick_test.php
The fastest usb sticks writes 800-1000 mbps in the crystaldisk-test.
If you have personal experience on this, please recommend which approach is better.
USB flash drives usually use the worst flash memory available. There are good ones but they are hard to find and expensive. Going for a NVMe SSD in an USB-C enclosure gets you way more reliably better speeds and durability.
A USB stick will be fine for ventoy unless you are using persistence and writing lots of data to it.
An NVMe drive in an enclosure is better if you don't mind the larger size and higher cost. A good NVMe drive will contain higher quality flash memory and have better wear leveling than a USB stick. There are USB 4 and thunderbolt enclosures available that can transfer over 3 GB/s.
Reminder to check the chipsets on your enclosures. Return anything made with JMicron, they're crap and will overheat under long transfers.
If you use a usb 3.2 gen 2 port, your max speed will be 1250 MB/s, pretty much every nvme ssd will be able to max that. Fast USB sticks will undoubtedly be very close, but might not have the same sustained write speeds compared to an SSD (With Dram cache).
If you're just going to use it for recovery ISOs or installations it's probably not going to matter much, I regularly use USB 2.0 usb sticks for that purpose just fine.
It's up to you though, I had an nvme ssd laying around and bought an enclosure for it, I get about 1000 MB/s read and write with it.
So if you have an ssd laying around I think that would be a good option, but a usb stick will be fine as well and would be a little bit more compact, if that matters to you
I have a 32gb USB flash drive I got from Protectli with some other purchases I mad (it was cheap and is tiny and metal) I was surprised how fast it was, so I am using it for my ventoy boot disk and have 15+ isos on it.
I actually just used it last night and copied windows10, debian 12.5 and Linux mint isos
They all copied pretty quickly!