In multiplayer, a block change next to an unloaded chunk that changes the opacity level of the block causes a massive client freeze, about 1-2 seconds for a single block.
A light level change next to an unloaded chunk also causes a freeze, but a much shorter one. Around 10-20 blocks have to change their light level for the freeze to be noticeable.
A block change that does not affect the light level or opacity level does not appear to cause any freeze.
These freezes only seem to happen when connected to a server, not in single player. I believe these are the causes of most common freezing in custom PvP maps, which are often slightly larger than the view distance and have many block changes happening everywhere, all the time.
The following video demonstrates the freeze from a single block opacity change. Notice that the light level of the block is zero, and the block is completely enclosed in other opaque blocks: