Spring Boot Reactive: elapsed time calculation - spring-boot

Question

I am currently using Spring Boot Reactive (using webflux) to develop a microservice. In it, I implement some kind of elapsed time calculation to determine how long it took for the process to run.
Basically, when the process start, I initialize the current timestamp to mark as the start timestamp as follows:
...
metrics.setStartMillis(System.currentTimeMillis());
...
and then it will print whether the process success or not along with the elapsed time in doOnSuccess() and onErrorResume() respectively:
...
metrics.setStartMillis(System.currentTimeMillis());
return webclientAdapter.getResponse(request)
.doOnSuccess(success -> {
metrics.info(true); // This will print a metric log indicating a success process along with process time with current timestamp
})
.onErrorResume(error -> {
metrics.info(false); // This will print a metric log indicating a failed process along with process time with current timestamp
});
...
When testing the service by mocking the backend call with a 100ms delay using cUrl, the elapsed time manage to be printed correctly (~100ms elapsed time), however, during a load test using jmeter, the printed elapsed time become very fast (~0 - 20ms ish) although the service is configured to call the mock backend with 100ms delay.
Does this have to do with the nature of Reactive being in an event loop, and if so, how can I ensure that the calling process elapsed time is able to be calculated properly?
Pardon if there is any confusion, feel free to ask additional information.