This paper presents a novel method for fast and quantified estimation of the Perceived Quality of Service (PQoS) for MPEG-4 video content, encoded at constant bit-rates. Taking into account the instant PQoS variation due to the Spatial and Temporal (S–T) activity within a given MPEG-4 encoded content, this paper introduces the Mean PQoS (MPQoS) as a function of the video encoding rate and the picture resolution, and exploits it as a metric for objective video quality assessment. The validity of this metric is assessed by comparing PQoS experimental curves to the theoretical benefit functions vs. allocated resources. Based on the proposed metric, and taking into account the qualitative similarity between theoretical and experimental curves, the paper presents a prototype method for pre-encoding PQoS assessment based on the fast estimation of the S–T activity level of a video signal.