Immersive virtual environments are increasingly investigated as tools capable of modulating conscious experience, yet the specific contribution of graded immersion to altered states of consciousness (ASC), time perception, and cognition remains unclear. The present study examined how different levels of immersion during videogame play influence subjective experience and post-experience cognitive performance. Seventy-two participants played an identical 35-minute segment of the videogame Half-Life: Alyx under one of three conditions: desktop PC (low immersion), head-mounted virtual reality (VR; medium immersion), or VR combined with full-body locomotion via an omnidirectional treadmill (high immersion). Following gameplay, participants completed validated measures of presence (IPQ), immersion (IEQ), ASC (5D-ASC), retrospective time estimation, and cognitive flexibility (Stroop task and Alternative Uses Test). Presence was selectively enhanced in VR relative to desktop play, whereas immersion was highest in the VR plus treadmill condition. Specific ASC dimensions related to embodiment and self-experience (disembodiment, depersonalization, derealization, and altered perception of time and space) were significantly elevated in immersive conditions. Retrospective time estimation accuracy was reduced in the highest immersion condition, indicating increased temporal distortion. Cognitive flexibility measures showed no broad modulation by immersion, with only subtle differences in Stroop accuracy. Overall, the findings indicate that increasing immersion during videogame play selectively reshapes specific dimensions of conscious experience, particularly embodiment- and time-related aspects, without globally altering executive function.