Why performance diffs between Flux.range and Flux.range().flatmap(Flux.range())? - spring

Question

I'm using spring webclient based on netty to initiate massive http requests, assuming:
total = page * cnt
here total is the total request.
One option is to use Flux.range(1, total) as:
#RequestMapping(value = "/tid1", method = RequestMethod.GET)
public Mono<String> total(#RequestParam(value = "total") Integer total) {
long ms = System.currentTimeMillis();
return Flux.range(1, total)
.flatmap(n -> {
// WebClient POST request here
})
.then(Mono.just(total))
.map(n -> {
long ms1 = System.currentTimeMillis();
long du = n * 1000 / (ms1-ms);
String res = "Num: " + n + ", QPS: " + du;
log.error(res);
return res;
})
;
}
Another option is splitting total into page pages, each page takes cnt requests:
#RequestMapping(value = "/tid", method = RequestMethod.GET)
public Mono<String> paging(#RequestParam(value = "page") Integer page, #RequestParam(value = "cnt") Integer cnt) {
long ms = System.currentTimeMillis();
return Flux.range(1, page)
.flatMap(n -> {
return Flux.range(1, cnt)
.flatMap(k -> {
// WebClient POST request here
});
})
.then(Mono.just(cnt * page))
.map(n -> {
long ms1 = System.currentTimeMillis();
long du = n * 1000 / (ms1-ms);
String res = "Num: " + n + ", QPS: " + du;
log.error(res);
return res;
})
;
}
I found that the performance diffs a lot between these options, the first one has a QPS of 12500, and second has a QPS of 7000(QPS is the requests number processed in one second).
Unfortunately I must adopt paging solution, and I wonder why these two approachs has so much difference?