World Composition, does it take a lot of resources?

Hey everybody, this is more like an understanding question.
I am using a 64m² landscape which I can edit without World Composition in no time.
Placing Foliage (whole map at once), aplying layers seems to be done within seconds.

But when I do the exact same things in the same map with World Composition (256) tiles it takes forever.
Meanign that it takes about 5-10 minutes to apply a single layer… somtimes I even get “Out of video memory”.
I am wasting so much time to edit the map… it’s insane. The original plan was to use a 256km² map, which I couldn’t even load all tiles without crashing…(it was 1024 seperate tiles).

So I would like to understand what happens behind the scenese with world composition so I can fix and act accordingly. Leaving world composition behind and just go with one single landscape seems to be not a good idea. With a single Landscape I get about 20-25 % more FPS drop.

My System Specs:
16 Core (32 Thread) @4GHZ CPU
32 GB 3000MHZ RAM
1TB 970 EVO M.2 SSD

Reducing the Tile count drasticly improves the performance when building / placing etc.
I reduced the tiles to 16 in total on a 8km x 8km landscape. Before I had 256. Is this the way to go?