There is advice to locate the packages(addons?) to a drive where one has more space. I intend to install the game to same drive as windows where I have at the moment luxurious 500Gb free for this project. Now, won´t it slow down the gaming considerably if the packages are on a other drive? Especially so as the main is SSD and the other normal HDD.

First off, "packages" in MSFS may refer to either the official content or the "community" add-ons.  During the installation you get the choice both of where to put the app itself and, a bit later, where to put the packages.  Using separate drives should not slow down the simulator (it may even speed it up slightly if the program needs to access both at the same time).

However, the packages form the bulk of the simulator both in terms of gigabytes and in terms of the need to access them.  Thus putting the packages on an HDD will definitely slow down the loading process at least.  I don't know if it affects the in-game speed as I use an SSD.

A half-way house, if you have enough space on your C drive, would be to install both the simulator and the packages there - during installation this means just the official content - and then put your addons somewhere else and use the MSFS Addons Linker to link them to the simulator.  The addons then won't use up space on your C drive, but the speed will still suffer if they're on HDD.   Another option the Linker gives you is to put all your add-ons in their own folder in your C-drive for now and then move them to your HDD (or a new SSD) when your C-drive fills up.  You can even have multiple folders for the addons (including on different drives) but that may be excessively complicated.  Either way, the Addons Linker is the best way to manage your add-ons and definitely a must-use.

As a guide, my MSFS install, including all the latest updates, consists of 8GB in the apps folder and 156GB in the official packages folder so you're looking at 164GB for the base install if you put it all on one drive.  This is likely to increase as Asobo release more official content (such as the GOTY edition and future World Updates).

Thanks a lot, it makes sense.