Postgres - should I keep the log file on the fastest drive
With the logging_collector enabled, writes are sequential so you can keep log_directory on almost any media.
Assuming average size of a log message is 5 KiB, with a HDD that can do sequential writes at 100 MiB/s you can log 20k messages/second.
So in theory - if you need to you emit tons of logs - you could put them on a separate, cheaper storage.
At the same time, remember that when the log volume fails or is overloaded, it can effectively stop the server (as noted in documentation linked above).