Check this video.
I assumed DJI optimized RTH speed to be the most effective and drain the least battery.
However this guy checked it and the auto RTH drained 7% more battery.
So it seems if you have low battery, it is better to kick into sport mode and come back manually on full speed, rather than on auto RTH speed. Interesting.

Thanks for sharing!

Flying in Sport mode takes a good deal more knowledge and skill of flying the Spark.  I would caution users to use their best judgement on their skill to fly in Sport mode before deciding this is the most appropriate way of getting their Spark back home in a critical battery situation.

Of course the best course of action is to try to keep yourself from getting into the situation in the first place.  But we all have done it, so just be aware of your skill to be able to pilot in Sport mode in a direct and efficient manner before deciding to use that method of bringing your Spark home in an unpredictable situation.  Lights flashing and beeps beeping can affect fine motor skills and judgement.

Kloo Gee Posted at 2017-10-5 12:06
Flying in Sport mode takes a good deal more knowledge and skill of flying the Spark.  I would caution users to use their best judgement on their skill to fly in Sport mode before deciding this is the most appropriate way of getting their Spark back home in a critical battery situation.

Of course the best course of action is to try to keep yourself from getting into the situation in the first place.  But we all have done it, so just be aware of your skill to be able to pilot in Sport mode in a direct and efficient manner before deciding to use that method of bringing your Spark home in an unpredictable situation.  Lights flashing and beeps beeping can affect fine motor skills and judgement.

"...can affect fine motor skills and judgement."  Thank you Darwin