Where is the difference?

What is the real main difference between raspbian OS and DietPi?
I understand you have made it more lightweight by cutting out installed applications.
But i wonder, is there any real tweaks that makes it faster and more optimized?

Yes DietPi is an extrem lightweight and slim/diet version of Raspberry OS

Due to the reduced amount of installed software, DietPi is starting much faster. Furthermore the mount of r/w operation has been reduced down to a minimum to allow a longer lifetime of SD cards.

Next point is the DietPi automation. If you select a software title from the catalogue, whole installation as well as configuration will be done by DietPi based on best practice. There is no need for users to preform any manuell step. It’s basically plug and play.

Last point is the DietPi script logic, supporting users as much as possible to configure and administrate their system, even with limited unix skills.

Plus Raspbian is specifically built and tuned for Raspberry Pi’s, DietPi with a tiny bit of tweaking can be run on a multitude of SBC’s

There isn’t as big a following of some of those SBC’s and only a handful of developers working on DietPi, however with that said those developers are making HUGE improvements all the time and are usually pretty quick about it

well on Raspberry Pi device, it’s still a Raspberry OS. On other devices, other OS versions like Armbian, Meveric or plain Debian are used. But this depends on the device model.