Just a thought: virtual assets are streamed into the engine; usually from disk. If you are streaming from a platter-drive it can be problematic, or even a slow SSD.
Best served with an M2-level drive as the I/O speeds are an order of magnitude(s) faster.
Also, if you do not have enough RAM to store the assets, memory management will start thrashing as it has to reorder the cache, free space for new stuff, etc and spend more on overhead vs processing.