  Measuring presence is critical to improving user involvement and performance
in Mixed Reality (MR). \emph{Presence}, a crucial aspect of MR, is
traditionally gauged using subjective questionnaires, leading to a lack of
time-varying responses and susceptibility to user bias. Inspired by the
existing literature on the relationship between presence and human performance,
the proposed methodology systematically measures a user's reaction time to a
visual stimulus as they interact within a manipulated MR environment. We
explore the user reaction time as a quantity that can be easily measured using
the systemic tools available in modern MR devices. We conducted an exploratory
study (N=40) with two experiments designed to alter the users' sense of
presence by manipulating \emph{place illusion} and \emph{plausibility
illusion}. We found a significant correlation between presence scores and
reaction times with a correlation coefficient -0.65, suggesting that users with
a higher sense of presence responded more swiftly to stimuli. We develop a
model that estimates a user's presence level using the reaction time values
with high accuracy of up to 80\%. While our study suggests that reaction time
can be used as a measure of presence, further investigation is needed to
improve the accuracy of the model.
