HAProxy community

Slow URLs using Multi-backends

Hello,
Could you please help me?
i’m using :
Two VMs Haproxy 1.8 (open source) on Debian9 (and reparting the load between them with CRMsh/pacemaker/corosync) each one 3 vCPU and 12Gb RAM
When i set just one URL in the Frontend ACL and one Backend server it works fine.
but when setting many ACL in one frontend for different URLs that goes to different webservers/ports on different backends , the URLS opens very very slow and sometimes i get “503 gateway error”.
i tried playing with timeout connection/timeout server/timeout client but didn"t solved the issue… i tried setting each app in different Frontend a part… the same issue persists…
all webservers are Microsoft IIS… here is my config:

global
log /dev/log local0
log /dev/log local1 notice
log 127.0.0.1:514 local0 warning
chroot /var/lib/haproxy
stats socket /run/haproxy/admin.sock mode 660 level admin
stats timeout 30s
user haproxy
group haproxy
daemon
# turn on stats unix socket
stats socket /var/run/haproxy.stat

defaults
mode http
log global
option httplog
option dontlognull
option forwardfor except 127.0.0.0/8
option redispatch
maxconn 80000

listen stats
bind *:9999
mode http
log global
maxconn 40000
stats enable
stats hide-version
stats refresh 30s
stats show-node
stats uri /

#----
#Front
#----

frontend front_INTE_QA
log-format “%ci:%cp [%tr] %ft %b/%s %TR/%Tw/%Tc/%Tr/%Ta %ST %B %CC %CS %tsc %ac/%fc/%bc/%sc/%rc %sq/%bq %hr %hs %{+Q}r”
http-response set-header X-Frame-Options SAMEORIGIN
http-response set-header X-Content-Type-Options nosniff
http-response set-header X-Proxy VMHAP1
mode http
bind *:80
bind *:443 ssl crt /etc/haproxy/certificates/ #no-sslv3
redirect scheme https code 301 if !{ ssl_fc } # redirect 80 -> 443 (for owa)
log-format “%ci:%cp [%tr] %ft %b/%s %TR/%Tw/%Tc/%Tr/%Ta %ST %B %CC %CS %tsc %ac/%fc/%bc/%sc/%rc %sq/%bq %hr %hs %{+Q}r”
maxconn 40000
timeout connect 1800s

acl acl_app1 hdr(host) -i app1.domaine1.com
acl acl_app2 hdr(host) -i app2.domaine2.com
acl acl_app3 hdr(host) -i app3.domaine3.com
acl acl_app4 hdr(host) -i app4.domaine4.com

use_backend back_server1 if acl_acl_app1
use_backend back_server2 if acl_acl_app2
use_backend back_server3 if acl_acl_app3
use_backend back_server4 if acl_acl_app4

#----
#Backend
#----

backend back_server1
mode http
balance roundrobin
server srv_myserver1 10.10.11.5:8089

backend back_server2
mode http
balance roundrobin
server srv_myserver1 10.10.11.4:8088

backend back_server3
mode http
balance roundrobin
server srv_myserver1 10.10.11.15:8045

backend back_server4
mode http
balance roundrobin
server srv_myserver1 10.10.11.29:8050