Async file upload server has 33% less throughput compared to the sync version

[Behrang Saeedzadeh]

Source code with Gatling tests here (WIP):
https://github.com/turingg/file-server

I wanted to compare the performance/throughput of an async file upload
servlet to a sync version. To do that, I intentionally configured Tomcat to:

* Use at most 2 HTTP connector threads
* Accept up to 1000 connections
* With a queue length of 0

<Connector port="8080"
protocol="HTTP/1.1"
asyncTimeout="30000"
connectionTimeout="30000"
acceptCount="0"
maxConnections="1000"
maxThreads="2"
minSpareThreads="2"
processorCache="1"
redirectPort="8443" />

However despite that, my async servlet (
https://github.com/turingg/file-server/blob/master/src/main/java/xyz/behrang/fileserver/b/AsyncUploadServlet.java)
has 33% less throughput compared to the sync version ((
https://github.com/turingg/file-server/blob/master/src/main/java/xyz/behrang/fileserver/a/SyncUploadServlet.java
):

# Async

---- Global Information
--------------------------------------------------------
> request count                                       5000 (OK=5000   KO=0
    )
> min response time                                    218 (OK=218    KO=-
    )
> max response time                                   2234 (OK=2234   KO=-
    )
> mean response time                                   674 (OK=674    KO=-
    )
> std deviation                                        486 (OK=486    KO=-
    )
> response time 50th percentile                        464 (OK=464    KO=-
    )
> response time 75th percentile                        773 (OK=772    KO=-
    )
> response time 95th percentile                       1890 (OK=1891   KO=-
    )
> response time 99th percentile                       2142 (OK=2142   KO=-
    )
*> mean requests/sec                                555.556 (OK=555.556
KO=-     )*
---- Response Time Distribution
------------------------------------------------
> t < 800 ms                                          3780 ( 76%)
> 800 ms < t < 1200 ms                                 525 ( 11%)
> t > 1200 ms                                          695 ( 14%)
> failed                                                 0 (  0%)
================================================================================

# Sync

================================================================================
---- Global Information
--------------------------------------------------------
> request count                                       5000 (OK=5000   KO=0
    )
> min response time                                      1 (OK=1      KO=-
    )
> max response time                                   1617 (OK=1617   KO=-
    )
> mean response time                                   207 (OK=207    KO=-
    )
> std deviation                                        333 (OK=333    KO=-
    )
> response time 50th percentile                         36 (OK=36     KO=-
    )
> response time 75th percentile                        150 (OK=150    KO=-
    )
> response time 95th percentile                        992 (OK=992    KO=-
    )
> response time 99th percentile                       1312 (OK=1312   KO=-
    )
*> mean requests/sec                                833.333 (OK=833.333
KO=-     )*
---- Response Time Distribution
------------------------------------------------
> t < 800 ms                                          4458 ( 89%)
> 800 ms < t < 1200 ms                                 479 ( 10%)
> t > 1200 ms                                           63 (  1%)
> failed                                                 0 (  0%)
================================================================================

Any ideas what am I missing here?

Thanks in advance,
Behrang