I couldn't find something on the internet, so maybe you can help me.
I run Debian + i3 on my laptop, and it takes incredible long, to start some software, especially from the Gnome-DE.
Does one of you know, why this occurs ?

Without any more information I don't think there is anyone who can tell you the specific reason.

It could be because of a hardware bottleneck (low ram, slow hdd, slow processor, ...)

It could also be because of some software in the background that takes up all the resources, random errors or unoptimized code, outdated drivers, ...

Just try to monitor all activity and maybe you'll find it

Maybe upgrade to an SSD? Clean up your autostart? ...?

Already got and NVME-SSD, none of my 4 cpu-cores is used more than 50%, and only 400mb of my 8GB ram are in use.
I also didn't have these issues, when I used a complete DE like xfce.

@metamourge You could try to reinstall some software. Also, make sure drivers and other dependencies are up to date

Gnome is heavy and slow, but even so, I think you probably have something else going on. Does anything stand out in top/htop?

Also see if something is flooding /var/log/messages

And look at dmesg for obvious errors.