I am looking for a good tool to write .iso images on USB sticks. I only found "MultiWriter" in the official repo, which is terrrible slow. It asks me middle in the copying process for a password and terminates then with an error.

Is there a better alternative?

Balena Etcher has never failed me over the past few years, when I don't use the dd command. Rosa image writer is mentioned on KDE Neon's website, and I have not had issue with that one, either.
Neither of these are found in the repos, though.

mintstick work flawessly

https://pkgs.org/download/mintstick

tips: remember to install python-gi for a correct launch

mkusb has always worked flawlessly for me.
https://help.ubuntu.com/community/mkusb

etcher - have used it , it works great and is easy.

dd - have used it , it works just fine , BUT can be the most dangerous if you get the destination wrong , it gives NO warning just starts writing to the destination and will happily overwrite whatever it was .

mkusb - have used it , works great , most versatile as it will also create live + persistence + windows storage (ntfs partition) , or not

VINNY

multisystem >>>> its the best

sudo apt-add-repository 'deb http://liveusb.info/multisystem/depot all main'

wget -q -O - http://liveusb.info/multisystem/depot/multisystem.asc | sudo apt-key add -

sudo apt update

sudo apt install multisystem -y

unfortunately with linux the usb key maker depends on the os you are wanting to make a key for, unlike windows you just simply use rufus and it will correctly setup whatever os you throw at it. if a windows program can do that there is no real reason one single program for linux can.