I don't have such a large assembly in direct modeling mode, I assume thats what you're asking for. My biggest assembly has about 400 components but it's designed with the timeline on. It also was an iteration of an earlier design and I had a clear understanding of what I wanted the assembly/browser structure to look like. This is performant even on my 7 year old i7 iMac.
However, if a design is more exploratory or complex reorganizing the assembly structure might become necessity. Reorganizing the assembly structure in a timeline model such as move this component into that assembly or move that subassembly in to this assembly I find is a nightmare and it can easily break the timeline due to the parametric dependencies.
Ive helped many Fusion 360 users with their designs and assemblies including a few with 1000 components and all to often it is clear the the timeline can quickly turn from a blessing into a curse and unfortunately that clearly manifests itself in performance problems.
However, if your design such things in direct modeling mode, such limitations do not exist. A friend of mine (another Expert Elite) designed a train model with 16000 (Yep, no typo) components. That number of components would be entirely impossible in timeline mode!
Yes, if it comes to assemblies Fusion 360 is not very efficient due to missing functionality and lack of performance.