Hi @TomasL44.
If you don't see any frequent geometry updates and changes, then yes, you could make same-category-per-floor detail features into a single-multipart feature.
Other load-reducing methods would be;
1. Set visibility scale (ex. features not to be displayed beyond 1:700)
2. layers as image service
3. Vector Tile (does not support floor-aware at a current release. Ideal for single-floor data only)