I think you may have identified one of the main contributing factors here… I would suggest that system RAM for the PC greatly influences performance with some of the add ons available. (Personally I am not seeing performance issues between a vanilla sim and a full community folder- but I have 32 GB RAM).
I am pretty sure MSFS manages add ons pretty well, only actually loading the relevant scenery into memory when one is flying in the area that requires that scenery. I suspect user defined LOD settings combined with which add ons are being used could contribute massively to performance, more than the actual size of the community folder.
Load times are definitely affected by larger community folders.
One other thing that seems to be a “feature” these days, is possibly related to performance. It appears that MSFS has recently introduced some sort of hard limit for injected “objects”, e.g. AI traffic (whether live traffic or from one of the 3rd party addons, e.g AIG or FSTL) and also injected ships/boats. I am not sure of the exact number of allowable injected “objects” but it appears to be fewer than what it was before, which may explain a reduction some people are seeing in live traffic- if there are nearby injected ships etc. I think this is also the case for 3rd party airport scenery that has lots of static “sim objects” to populate parking stands. An airport full of these can reduce the ability for other add ons to also inject “objects”.