Introduction Immersive virtual reality applications are increasingly popular in entertainment, education, and professional training. While many aim for maximal realism, simplifying the virtual environment may offer benefits such as reducing mental workload and improving focus on core tasks. However, the impact of different types of demand on users’ mental workload remains unclear. Objective This study explored the impact of visual, auditory, and cognitive demands on users’ mental workload during a daily living activity in immersive virtual reality. Methods Twenty-four participants used a head-mounted display for a virtual shopping task, i.e., picking ten listed products from a shelf, under different conditions: visual demands (moving characters), auditory demands (background noise), cognitive demands (simultaneous arithmetic task), and a combination of all three. Mental workload measures included heart rate, pupil diameter, and self-reported mental demand & effort. Results The cognitively demanding secondary task induced the largest mental workload, significantly exceeding that of auditory and visual demands. For example, on a scale of 1 (low) to 10 (high), self-reported mental demand & effort was 4.40 for the moving characters, 5.00 for the background noise, 6.67 for the arithmetic task, and 7.17 for the combined condition. Biosignal differences were consistent within participants but were masked by high inter-individual variability. Conclusions In virtual shopping tasks, reducing enforced cognitive demands may be more effective for decreasing mental workload than reducing non-task-relevant visual or auditory demands.