AE's default lossless output module generates an intermediate file that is designed for additional post-production use, not direct playback. These files are generally huge, with very high data rates, and very few computers can play them back in real time without stuttering. For more, see this Adobe FAQ [link]: Why is my output file huge, and why doesn't it play back smoothly in a media player?
To help you choose a more suitable format for your needs, see the related Adobe FAQ [link]: What is the best format for rendering and exporting from After Effects?
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