Resolution: Works As Intended
Affects Version/s: Minecraft 14w30c, Minecraft 14w31a, Minecraft 1.8, Minecraft 1.8.1
Fix Version/s: None
This problem started after the 14w28-29 Updates. I will do my best to explain as its a complicated idea.
Fill clocks using 2 command blocks doing opposite fill / replace commands to run a every tick redstoneblock clock, cause local ( in game position local ) Client FPS lag, as well as severe Network bandwidth Lag ( again locally to the position of the player ).
Client Video / FPS Performance.
When you have a certain amount of command blocks running off the clocks seems to be around 50 the client starts to perform a massive amount of chunk updates per tick 300ish from my testing. When facing the opposite direction in game as the location of the fill clocks the chunk updates and FPS performance subsides. This effect happens even if Client does not have line of sight to the clocks themselves ( putting the clock in a closed space or having a wall between the user and the clocks.
Running the same clocks in a pre 14w28 snapshot, and running them in current snapshot you can see a VERY clear difference. in the new snapshot there is a seemingly linear relationship to number of command blocks running on the clocks to amount of bandwidth being used. Running about 50-60 command blocks basically makes the game unplayable through a DSL connection ( If your character is in the same loaded chunk area as the clocks ). If your teleported away from the clocks the network action subsides. The direction your character faces or anything to that effect have no effect on the network bandwidth increase seen by these clocks.
The reason I am saying this is a bug is because it has changed greatly in the latest snapshots, and seems to make it nearly impossible to have any larger number of command blocks running in a multiplayer server, which is something I would suppose is not out of the realm of intended use.
Hopefully I have explained this clearly, I can provide some video evidence and statistics of the tests I have done to come to these conclusions if that is helpful.
I really hope this will get resolved as doing some multiplayer command block stuff on servers is more or less not possible anymore.