Registry synchronisation

It says in the docs that doing reg operations without flushing can delay the changes several seconds. Does this mean if I write to the registry then immediately read from it it may read the old value, or does the delay only affect other programs that try to read right after I wrote it? (it will in fact be synchronized in my program, using a cache or something)

I think that just means the change being committed to disk would be delayed (are you planning a power cut?)
As for running code, I would expect the change to be immediate via the cached registry entries in memory.