This is the 7730th most frequent French word.
slo
"Slo" is not a word in standard French.
Usage of 'slo' replaced with 'lentement', its French equivalent, meaning 'slowly'.
Je marche lentement sur le chemin pour ne pas trébucher.
I walk slowly on the path to avoid tripping.
Here 'slo' replaced with 'long', referring to the 'slow' nature of the process.
Le processus est un peu long, mais il faut être patient.
The process is a bit slow, but patience is required.
The sentence uses 'lente' describing the 'slow' speed of the turtle.