I need to install Ubuntu and Kali Linux on USB Sticks. Before that, what's the minimum requirement that a USB stick should have (for best performance)? For example, Read/Write speed.
Minimum requirements for the size of the USB flash drive are 4GB for Ubuntu 18.04-22.10 and 8GB for Ubuntu 23.04 and later. Minimum requirements for the size of the USB flash drive are 2GB for Ubuntu 14.04 and 4GB for Kali Linux (regular). A USB 3.0 drive will be faster than a USB 2.0 drive, but still much slower and less stable than running either OS in a virtual machine application like VirtualBox if you allocate at least 2GB RAM to the guest OS.
If you already have access to a running instance of Ubuntu, the Startup Disk Creator application that is built-in in Ubuntu is the best application to use for making a persistent Ubuntu live USB (however the persistent storage feature no longer exists in Startup Disk Creator in Ubuntu 16.04 and later). It seems to boot successfully on more different machines than if other alternative applications are used for making the Ubuntu live USB. Keep this in mind if you run into problems booting the Ubuntu live USB that you made using some other app.
If you are using Ubuntu 16.04 or later, you should also read sudodus's comment below this answer.
