As a beginner of SLSB EJB, I understand that accessing them locally (through Local Business Interface) is more efficient than remotely.
When an EJB is accessed through an Endpoint, is it being accessed remotely or locally?
This is in the context of accessing the EJB through JAX-RS.
A webservice endpoint is neither local nor remote. It is its own client view.
That said, the overhead from remote is primarily due to marshalling/demarshalling arguments for pass-by-value semantics of parameters and return types. The webservice views will require the same, so you should expect slower performance than local, which uses pass-by-reference. Whether or not a webservice view is faster or slower than remote depends on the webservices and remoting implementations.