Abstract: 1.7.4 causes more CPU load on a server than 1.7.2 did. This was tested by tracking server CPU usage under varying player conditions, then downgrading both server and client.

Details:
I'm the Admin of a small private survival server. No game-mods are used. We updated to 1.7.4 the day it was released. The server is a purchased plan from an online provider. I'm trying to get the technical specs. The server is rated for 16 players in Vanilla.

We noted unusual amounts of "block-lag" (delayed chunk updates between client and server), difficulty interacting with doors and chests due to latency, very slow loading of new areas, this sort of behavior. Originally we thought it was network latency. Then, the server began occasionally crashing whenever 3 or more people were on.

I investigated through the server management tools provided by the service provider. One detail this displays is the CPU usage, represented by a percentage, where "100%" represents full use of a single server CPU, and >100% means we're using multiple CPUs.

I found that with 1.7.4 installed the CPU would sit at 47% usage with only one user, sitting stationary in the server Spawn Chunks, with no redstone clocks or hopper sorters running, after hunting and killing all egg-holding zombies. One user travelling outside the spawn chunks in the overworld produced CPU usage between 80% and 98%, depending on what was in the area. Travelling to the Nether from outside the overworld spawn chunks caused 120% usage that lasted until the overworld "shut down".

Multiple users travelling or working would cause sustained 150% usage, which caused the server to kick all players and restart, which to an uninformed player appeared to be a crash.

Last weekend we down-graded to 1.7.2. This reduced the load of 1 user idling in the spawn chunks to 25% usage, traveling outside spawn reduced to 35% usage, transit to nether from outside overworld spawn chunks reduced to 60-65%. Multiple users can now be online without "block lag", and we have tested having one user in the nether working while another user is in the overword farming 550 cave spiders 1km away from spawn, which produced a server CPU load of only 75%.

Conclusion:
I don't know what caused this, I just have seen that downgrading from 1.7.4 to 1.7.2 has substantially reduced server load under all normal conditions, without changing the server in any other way. Even though the client 1.7.4 version is compatible with the server 1.7.2, all users have also downgraded to 1.7.2 to accommodate the change. This testing was completed AFTER hunting and killing all egg-carrying zombies under Spawn, and no other changes were made to Spawn during testing.

Please let me know if there is any other data you would like, I'll do my best to provide it. I'm not sure what data is available in our server logs, I'll check those tonight.