flatMap on Mono<byte[]> takes a long time to execute

Question

We have a function in our Spring webflux application which takes in a Mono<byte[]> as below.

public Mono<VirusScanResult> scanFile(Mono<byte[]> bytes) {
    int RETRY_COUNT = 1;
    return bytes
        .flatMap(this::scanFileBytes)
        .doOnError(throwable -> log.error("Scanning failed with error", throwable))
        .retryWhen(Retry.fixedDelay(RETRY_COUNT, ONE_SECOND));
  } 

This function is in the service layer of a REST endpoint which accepts Mono<byte[]> as below

public Mono<VirusScanResult> scanFile(
      @RequestBody @Schema(type = "string", format = "byte") Mono<byte[]> data) {
    log.info("Virus scan requested");
    return virusScanService.scanFile(data);
  }

The problem is that as the byte[] grows bigger with bigger documents being sent in the bytes.flatMap in the first code snippet above takes a lot of time since it iterates over the huge byte<>. Is there a way to extract the byte[] from the Mono<byte[]> without using flatMap and which is more effecient. At the moment when a 25MB file is sent to the Rest API, bytes.flatMap code takes around 13 seconds to execute

I also tried with code snippet below with Mono but again it takes time at filePartMono.flatMap

public Mono<VirusScanResult> scanWithFilePart(@RequestPart Mono<Part> filePartMono) {
    log.info("scanWithFilePart Controller started");
    return filePartMono
        .flatMap(
            fp ->
                DataBufferUtils.join(fp.content())
                    .map(dataBuffer -> dataBuffer.asByteBuffer().array()))
        .flatMap(bytes -> virusScanService.scanFileBytes(bytes));
  }

Kindly advise