What's the point of encryption with 3x3 in OnePlus 5 if it has 409104 combinations meaning it can be cracked under a second with any modern processor? I am not sure what I am missing, but decryption the sdcard should be as easy as opening it if it it that few possible combinations. So what's the point of it?

To clarify I mean booting into recovery mode, ADBing, and trying to decrypt the partition directly, without artificial timeouts that OSes do.
Countering your supplemental conditions...


  To clarify I mean booting into recovery mode, ADBing, and trying to decrypt the partition directly, without artificial timeouts that OSes do.



Stock recovery doesn't have regular ADB capability (only ADB sideload, and that only flashes signed firmware).
Even with USB Debugging enabled, you can't authorize a random PC to debug over ADB, since the authorization dialog is behind the lockscreen.
Can't read/pull anything from /data even if you have ADB authorization, unless the device is rooted.
Can't flash a custom recovery without unlocking the bootloader, which nowadays requires flipping a switch in Settings - Developer options, again behind the lockscreen. Unlocking also fully wipes all data including the internal storage.
If the bootloader is already unlocked then theoretically none of the security measures would ever matter - but such is a very rare case for regular users, and anyone who do it would (or should) know the risk well already.