Whats the slowest shutter speed you can shoot to achieve useable images?

I decided to find out at the local MX track. These photos were all shot on a Nikon D750 using a 25 year old Nikon 80-200mm F2.8 lens. The lens nor body has any form of stabilisation or vibration reduction. All these shots are hand held which means no monopods/tripods.

The useable images i came away with were taken using a shutter speed of between 1/25th sec to 1/50th sec at 200mm focal length. This is very slow for a 200mm lens of this age and not advisable if you want a great hit rate of sharp in focus images. I had never tried to shoot with a shutter speed this slow on this lens and so I was pleasantly surprised to find that many of the shots worked and had the effect i was aiming for. What I wont tell you is how many shots were complete rubbish and blurry/ out of focus ( Hint- It was A LOT ).