If you don't want to code a json parser (/interpreter), this is probably the fastest solution. Just make sure instead of using stuff like push, better resize the array at start of the merge and then just set at - I think this should be more memory-efficient (not sure tho).
Other idea would be getting a json-expression out of the exisiting array and merge the two json strings correctly together and after that set the array from the single expression, but I doubt thats any faster, so no reason to do that.