Kali installation very slow

[Zehl]

Hi I am having a problem installing Kali and can't find a solution anywhere.
So I downloaded the latest Kali amd64 ISO and wanted to install it to a USB instead of adding persistence with the persistence.conf method.
The problem is that once it gets to the installing system part it takes hours and I mean really a long time. I left it once for 8 hours and it wasn't even at 50%.
I tried with encryption and then without thinking that was the problem, no change. I tried it in different computers and on virtual machines, even different USB ports. I also tested the USB on windows thinking it was reaching the end of it's write cycle and was loosing speed but windows can write all its blocks at full speed.

So I'm wondering if the current version of Kali has any known problems with the USB driver or if anyone has had this problem.

Oh my USB is a PNY 16GB.

[Zehl]

Ok so I was able to install it to the drive after many hours and the system is very slow almost unusable. I thought of creating a ramdisk and copying the system there on boot and back on shutdown but that's not very safe for the data on ram.
I thought of another solution but i don't know how to do it.

The reason I wanted to install it to USB was that I wanted to be able to update the distro and install stuff to it without breaking the loader when the kernel is updated, with the persistence.conf method that can happen if the live bootloader doesn't support a kernel update, also because I wanted to be able to use the system on other computers when needed.
So my solution is install the system to the hdd, the boot to the usb and then on the second partition of the usb the live cd. I really don't want to mess with the windows bootloader on the pc..
What I don't know how to do is pointing the grub on the usb to the live system so I can choose at boot time.
Does anyone know how to do this?