One thing to know is that data is never stored "in clear" in a SSD because storing random data (instead of long series of zeros and ones that are naturally present in a lot of files) has benefits for the SSD endurance and reliability. Older SSD were using LSFR but most (all?) newer SSD are using AES encryption to randomise the data. If you don't set a password the SSD is still encrypted but the key is "publicly available".
For more information, and sources, see Journeyman Geek's answer at
Why does my SSD internally encrypt data, even without a password set?
To secure erase the deletion of the encryption key is enough, and that's pretty much instantaneous. A TRIM of all sectors will also be done for wear leveling purposes, that's what takes your 3 seconds.
When you check that the sectors are really blank they are most probably not even read, there is no need from the controller point of view as you told it to TRIM them so it knows they are blank.