We already blog a lot on temporary tables on disk, how this could be bad for your workload and should be avoid. Each temporary table will open a file descriptor, external kernel call and by nature a well know file system slow operation.
We can point this benchmark simulating a working mail server
Monitoring of such queries can be trace via the status of
created_tmp_disk_tables Evidence of what is happening watching many Aria or MyISAM tables created on disk :
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