Although speedtest-cli --json is requesting json output, the exec node delivers it into Node-red as a string, so the json node just converts it back into json.
I have not changed any settings for this node, all default.
Here's the flow
[{"id":"f77909d8dd071c67","type":"inject","z":"32c594e46f312b73","name":"","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"","payloadType":"date","x":100,"y":40,"wires":[["467c01c066a58cb2"]]},{"id":"467c01c066a58cb2","type":"exec","z":"32c594e46f312b73","command":"speedtest-cli --json","addpay":"","append":"","useSpawn":"false","timer":"","winHide":false,"oldrc":false,"name":"","x":270,"y":40,"wires":[["a20245501a5ef6c3"],[],[]]},{"id":"408a0048a4cee966","type":"debug","z":"32c594e46f312b73","name":"output","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","targetType":"msg","statusVal":"","statusType":"auto","x":590,"y":40,"wires":[]},{"id":"a20245501a5ef6c3","type":"json","z":"32c594e46f312b73","name":"","property":"payload","action":"","pretty":false,"x":450,"y":40,"wires":[["408a0048a4cee966"]]}]