In PUBG, if the plane is going over a location you want to land at, is it better to leave early and glide your way there or wait to be over the location and go straight down?
To reach your landing spot the fastest you want to leave the plane about 200m before your target. Once you have left the plane hold forward and look straight down, this should point your character straight at the ground and have you descend at around 234 km/hr (65m/s) with very little forward movement. At around 300m from the ground your chute will open, continue holding forward, and point towards your intended landing zone. If you over shoot by a bit don't worry, as spiraling while holding forward has the same vertical speed as going straight. This means it won't affect how long it takes to hit the ground if you need to turn around last minute. On average your descent will take around 35 seconds.
Things get a little more tricky if your target isn't within 200m of the plane path, but the video below does a good job at breaking down player speeds/movement while free-falling and parachuting:
For reference here are the vertical and lateral speeds discussed in the video: