I think the delay should be rate limit 1 per 15 seconds, not delay
e.g.
[{"id":"5b2879f291e75a35","type":"inject","z":"da8a6ef0b3c9a5c8","name":"","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"","payloadType":"date","x":100,"y":4680,"wires":[["395a7600299bef98"]]},{"id":"395a7600299bef98","type":"function","z":"da8a6ef0b3c9a5c8","name":"create test array","func":"let count = 0;\nmsg.payload = [...Array(150)].map(() => {\n count++;\n return count;\n});\nreturn msg;","outputs":1,"noerr":0,"initialize":"","finalize":"","libs":[],"x":260,"y":4680,"wires":[["a47d1a71fb835c99"]]},{"id":"a47d1a71fb835c99","type":"split","z":"da8a6ef0b3c9a5c8","name":"","splt":"\\n","spltType":"str","arraySplt":"10","arraySpltType":"len","stream":false,"addname":"","x":430,"y":4680,"wires":[["4fdf9ca73fc4af33"]]},{"id":"4fdf9ca73fc4af33","type":"delay","z":"da8a6ef0b3c9a5c8","name":"","pauseType":"rate","timeout":"5","timeoutUnits":"seconds","rate":"1","nbRateUnits":"15","rateUnits":"second","randomFirst":"1","randomLast":"5","randomUnits":"seconds","drop":false,"allowrate":false,"outputs":1,"x":580,"y":4680,"wires":[["930b846aad428369"]]},{"id":"930b846aad428369","type":"debug","z":"da8a6ef0b3c9a5c8","name":"debug 233","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","statusVal":"","statusType":"auto","x":730,"y":4680,"wires":[]}]