There is no faster RESTful API calls for OpenDACS. In cases when the performance is not sufficient, or not desirable per design, the call is not made on every subscription request to check, rather a cache of entitlements is created, and maintained up-to-date, and checks go against the cached entitlements. The fastest way of getting entitlements is against a pre-populated and maintained cache of entitlements.
If you are considering/designing the caching mechanism, would like to suggest two articles that are helpful in understanding relevant OpenDACS concepts:
An Introduction to the DACS Entitlement System for OpenDACS Developers and
Open DACS and Open DACS Permission Server
Depending on your level of comfort with DACS, that will be implied, you may wish to additionally read An Introduction to the DACS Entitlement System for OpenDACS Developers