Quasi-synchronous systems aim to reduce energy consumption by allowing timing violations in a synchronous circuit, while performance guarantees are provided by analyzing the system with a suitable deviation model. This paper studies the performance of quasi-synchronous LDPC decoders for regular codes of finite length. We present an approach to accurately predict the decoding performance and energy consumption of the decoder for a specific average channel quality and maximum number of iterations. These analytical results are then compared with gate-level circuit simulations of a quasi-synchronous decoder.