Is the lock screen secure?

They could try to brute-force your password, you might want to set it up so accounts are locked out after too many failed attempts. You didn't ask how, but if you're interested you can read more here https://web.archive.org/web/20190831173642/http://blog.bodhizazen.com/linux/ubuntu-how-to-faillog/

Edit : including steps as requested

Open /etc/pam.d/common-auth and add the line AT THE TOP OF THE FILE:

auth required pam_tally.so per_user magic_root onerr=fail

To set the number of attempts allowed & timeout

faillog -m 3 -l 3600

Where 3 is the number of attempts allowed and 3600 seconds (1 hour) is how long to lock out the account for.

You can omit the -l part and the account will be locked out forever, however I would really not recommend that since your hard drive is encrypted. It would make your files very difficult to recover if you locked yourself out. If you choose to omit the lockout time, I would at least increase the number of attempts, because it's not that difficult to enter your password wrong 3 times.