If this is a 100m sprint start to finish.
Then setoff is way, just keep the speakers equal for each person if you need to be that accurate.
First to finish, you use a laser beam system and one it breaks you have you fastest time.
From then you can start recording (you could even have a pre laser beam to start pre-recorded.
I believe Camera and RFID are then the way to go.
If it was the Olympics anyway, something I read/was on TV, can't remember.
If it is just one runner, then a beam break for timing and an RFID tag for identity.
If this is sprint laps and starts are all over the place....
Just thought I'd do the old Google search to see what it comes up with and this one looks interesting.
https://www.quora.com/Usain-Bolt-sprint ... -precision
Though have a go, grab some tags and a laser break and just get them working. Then get so rudementary timings, they'll be no worse then someone with a stop watch.
Then build on that.
2000fps camera detection though..
Stick a Pi ZeroW and not a Pi3 as power usage is lower.
Link via WiFi as you can set one up as an Access Point and it make slide easier.
Consider ZeroW for the sensors and a Pi3 in the middle as the AP and sever.
You can even then access the Pi3 from your phone.
Power is just a matter of large batteries.
USB batteries are quite large now.