I have my phone set up to enter a long code on boot (which goes to LUKS) but the lockscreen PIN is much smaller. The low entropy on the lockscreen doesn't matter so much as it is capable of restricting the number of tries, delaying after a certain number of failures, etc.
On the phone, you don't need to encrypt all of the file system (for better performance) but just the parts that hold user data.
Unlocking the screen and encrypted user data by "swiping a pattern" is not a big thing and takes not even a second.