How to reasonably allocate and schedule resources of wireless sensor system to maximum its potential capability has been an important area for research. In this paper, we focus on the Optimal Sampling Frequency Assignment (OSFA) in a real-time wireless sensor networks (RTWSN). An appropriate OSFA should both guarantee a good quality of real-time service and efficiently utilize the limited network resources as well. We propose a distributed optimization algorithm, called Lazy Schema (LySa), to obtain the optimal sampling rates of source nodes with low cost. The central idea is that redundancy reporting and constant adjust step size result in the excessive communicate overhead in iteration process. LySa adopts self-adaptive reporting rates and dynamically adjusts step size to reduce the traffic cost and accelerate the convergence of optimal solution. We have evaluated LySa together with related mainstream algorithms. The results demonstrate that LySa outperforms current state-of-art approaches in terms of low cost, high efficiency and scalability in RTWSN.