How can I make KSP run at half-speed?

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In one of Scott Manley's videos (Interstellar Quest I think), I heard him say that he was running the physics of Kerbal Space Program at half-speed so his computer could handle it. He then doubled the speed in post-production.
How can I halve the simulation speed of Kerbal Space Program, or any other game for that matter?
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KSP Stock Land Speed Record - Breaking 4000 m/s, 9000 mph
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Images: Andrea Piacquadio, Andrea Piacquadio, Maria Orlova, MÃdia