I may not understand the whole context, but if you're using PBKDF2 for key derivation, you don't want to use something that works "fast". You want whatever algorithm/iteration combination that is as slow as possible [1], but not too slow that it annoys the user.
The kdf should be as slow as possible for an attacker but fast enough for you. If you're using python and the attacker is using C, you can increase security with no usability loss by switching to C and raising the iteration count.
[1] on modern hardware, that is.