How fast does difficulty adjust?

How fast does the difficulty adjust when there is a huge increase in hash rate? Hypothetically, lets say somebody brings enough rigs online to increase the network hash rate by 20% does the difficulty instantly adjust to this? Looking at the difficulty chart at https://ubiq.darcr.us/ it appears that it adjusts gradually over time. I started thinking about this after seeing the hash rate chart here http://www.ubiq.cc/hashrate It shows pretty large spikes and then large drops.