The ipv4 routing cache is non-deterministic, performance wise, and is subject to reasonably easy to launch denial of service attacks. The routing cache works great for well behaved traffic, and the world was a much friendlier place when the tradeoffs that led to the routing cache's design were considered.

What it boils down to is that the performance of the routing cache is a product of the traffic patterns seen by a system rather than being a product of the contents of the routing tables.