Circular movement in 5th edition d&d

This came up in a discussion in my group about circular movement.

Do we treat this as moving diagonally or difficult terrain?

For a situation such as circling a target on a horse shooting arrows at it we fell should be treated as difficult terrain

One situation was mentioned was what if a ranger casts spike growth which has a 20 foot radius wants to completely walk around it. How long would it take? The circumference is about 125 feet and ranger base speed is 30

This came up as we wanted to know if we cast spike growth catch someone in the area of effect. If they try to move out from it and away from us can we circle around faster then they can move through it

Running through it counts as difficult terrain but what about running around it?