No, the call is going through ContentProvider, which happens over Binder IPC, which entails context switching and other I/O overhead. Compared to local file operations (java.io and friends) this is massive; compared to anything else your device does, it's not a big deal at all, and it's not CPU-bound.
Slow means it's probably burning a lot of CPU, and that is a problem for background tasks, especially on mobile.