What is the faster local storage engine/approach for json files

My app retrieve a geojson from an api. The size of the json file is of 2Mo for the moment, but it will grow up faster.
So I want to save this file on the device to speed up the lunch of the app.
I tryed localForage and it’s working fine, but it’s not as fast as I expected.

My question is : what will be the faster engine/approach to store en retrieve this json file on a dive ?

Thanks for your experience feedback.