Use of MBTA-performance requires an API key. This is a different key from the V3 API key. You can request a key or manage your keys at the original MBTA developer portal, or use the public key to get started.
The MBTA-performance API shares characteristics and resources with API-V2. API-V2 is deprecated, but the MBTA-performance API is not. It is expected that API key management will change in the future. Watch the MassDOT/MBTA Developers Google Group for details.