during high traffic time, web site become much slower while low traffic is ok. a typical case is a download server, when there are many concurrent connections to download, many or all users feel much slower. one cause may be the linux conntrack table is full.

#dmesg | tail
nf_conntrack: table full, dropping packet.
nf_conntrack: table full, dropping packet.
nf_conntrack: table full, dropping packet.

if you see above, then the issue of conntrack table full is confirmed.

#sysctl -a | grep conntrack
...
net.netfilter.nf_conntrack_max = 65536
net.netfilter.nf_conntrack_count = 68999
net.netfilter.nf_conntrack_buckets = 16384
...

#cat /sys/module/nf_conntrack/parameters/hashsize
16384

if nf_conntrack_count is close to nf_conntrack_max or even larger, the problem will occur.

#sysctl -w net.netfilter.nf_conntrack_max=655360
net.netfilter.nf_conntrack_max = 655360
#echo 163840 > /sys/module/nf_conntrack/parameters/hashsize

edit /etc/sysctl.conf, add following line:

net.netfilter.nf_conntrack_max=655360

edit /etc/rc.local, add following line:

echo 163840 > /sys/module/nf_conntrack/parameters/hashsize