About above calculation: If 5% of cross-shard activity is uniform, then 2.5% of all cross-shard messages are passing through the root shard, and there is a 40x scalability limit.
However, the load from 2.5% of all cross-shard messages is not the same as 2.5% of the total load of the system. If we assume that cross-shard messaging consumes 10% of the system load, then the root shard bears 0.25% of total system load, leading to a 400x scalability limit.