Historical problems

In practice, the perfect system is not realizable. All of these operations require time to propagate information and perform computations. In early VR systems, the total motion-to-photons latency was often over $100$ms. In the 1990s, $60$ms was considered an acceptable amount. Latency has been stated as one of the greatest causes of VR sickness, and therefore one of the main obstructions to widespread adoption over the past decades. People generally adapt to a fixed latency, which somewhat mitigates the problem, but then causes problems when they have to readjust to the real world. Variable latencies are even worse due to the inability to adapt [69]. Fortunately, latency is no longer the main problem in most VR systems because of the latest-generation tracking, GPU, and display technology. The latency may be around $15$ to $25$ms, which is even compensated for by predictive methods in the tracking system. The result is that the effective latency is very close to zero. Thus, other factors are now contributing more strongly to VR sickness and fatigue, such as vection and optical aberrations.