Has something changed recently in testnet? Today I've been seeing new blocks appear every 1-5 seconds instead of the usual 10-20 minutes
Testnet has a built in functionality that changes the difficulty to 1 if the mining process takes 20 minutes or longer.
This question explains the dropping of difficulty to 1.
So while the difficulty is 1, the blocks are being formed rapidly until the difficulty reaches to a value which takes 10 minutes.