In other words, the rate of slowing increases with chain size. In order for Moore's Law to resolve node scaling, node performance must remain linear in relation to the chain size. The low early rate is just a consequence of empty blocks. But since blocks started becoming full, there is a significant decline on a per tx basis. It should be getting faster on a per tx basis as the blocks become more full (just as in the early stage). But non-scalability in relation to chain size is dominating.