This is a Splunk query to measure REST API response time from the various rest URI’s in Splunk.
index=_internal sourcetype=splunkd_access source=*splunkd_access.log | rex "- - - (?P<Response_Time>.*)" | rex "\"(?<REST_uri>[^\"]+)" | table _time, REST_uri, Response_Time
Credit goes to somesoni2 on answers.splunk.com! Query found here: https://answers.splunk.com/answers/112073/splunk-query-to-measure-rest-api-response-time.html