Every time a large enough amount of blocks is updated (for example by the fill-command), the complete chunk is resent - including ALL NBT-data (even if none changed). This results in terrible performance for things like high-frequency fill-clocks (both CPU-load for encoding and bandwidth)
This behavior is especially useless for command-blocks, where the NBT-data are never needed (except when opening the GUI, see MC-79872 for a partial solution)