Checkpoint question

To improve the performance of PostgreSQL, resp. to avoid too frequently occuring
checkpoints, it is recommended to increase the number of wal files
(checkpoint_segments).
So I increased the number of checkpoint segments to be able to write a lot of
data into the database without interruptions due to checkpoints.
My question now is concerning the cost of a checkpoint if I have a lot of
checkpoint segments. At checkpoint time will there be checkpointed all the
information contained in all log files, lets assume that I have 40 checkpoint
segments for example, or will only some informations of the log files be
checkpointed?
If all log files are checkpointed, won't this take a lot of time, and if so
wouldn't it be better to leave the number of checkoint segments to a smaller value?
I'm not quite familiar with the checkpoint concept, so maybe I'm wrong with my
thoughts. Can anyone give advice/explanations?

Thanks Andreas

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