You are measuring the decision to be fast, not inattention: the Sustained Attention to Response Task does not measure sustained attention.

The Sustained Attention to Response Task (SART) has been widely used in psychological literature as a measure of vigilance (the ability to sustain attention over a prolonged period of time). This task uses a Go/No-Go paradigm and requires the participants to repetitively respond to the stimuli as quickly and as accurately as possible. Previous literature indicates that performance in SART is subjected to a "speed-accuracy trade-off" (SATO) resulting from strategy choices and from the failures of controlling motor reflexes. In this study, 36 participants (n = 36) performed a series of four SARTs. The results support the perspective of strategy choice in SART and suggest that within-subjects SATO in SART should also be acknowledged in attempting to explain SART performance. The implications of the speed-accuracy trade-off should be fully understood when the SART is being used as a measure or tool.