It's probably because different understanding of same terms. With "complexity" word we mean hardness to generate. It doesnt adds up more algorithms but character sets and hashing algorithms for PBKDF2 respectively.
A password with lower levels of string complexity might make sense --- 1Password's strong passwords can sometimes be rejected by crappy websites. A password that sacrifices cryptographic strength in order to save a few tens of milliseconds of KDF, though, makes no sense at all.
Also: really, you should not be using PBKDF2.
Or if your attack model allows that reading protected files off a user's device is less likely than the cloud-synced database being compromised, you might derive the master encryption key as KDF_fast(KDF_slow(password)+password)), where KDF_slow takes 5 minutes and is stored on disk, but KDF_cheap takes 5 seconds.