Fast Vocal Tract Length Normalization(VTLN) implementation?
VTLN is pretty old technology. There is a fast linear VTLN or basis fMLLR which is about the same thing. These days everyone is doing neural networks though, they provide more advanced speaker adaptation, at the same time they could be pretty efficient with quantization and proper weight pruning.

For example you can check DOMAIN AND SPEAKER ADAPTATION FOR CORTANA SPEECH RECOGNITION by Microsoft.

Best adaptation method depend more on the task you are trying to solve, on the resource constrains you have, on the model you are using.