Preemptible kernel impact

Since the old times of broadcom based Dlink DIR-320 I've used custom openwrt/lede builds with preemptible kernel.
That old guy totally freezed all user processes when kernel sirq code overloaded cpu routing heavy traffic (40 mbits could absolutely hang wifi/dns/ssh/... ) and watchdog fired. Start torrent download and you're rebooting after 15 seconds.
This was OK for 4.4 kernel.
But yesterday I built 4.9 snapshot for ar71xx and found severe degradation of iperf stats.

For example :

4.4 preempt : 170 mbits (acceptable)
4.9 preempt : 90 mbits (heavy drop)
4.9 voluntary : 110 mbits (heavy drop)
4.9 no-preemption : 220 mbits (normal)

So, is this by design and caused by changes in vanilla 4.9 kernel or its problem with lede patches ported to 4.9 ?
No wifi, just ag71xx. Measured with iperf running on router + iperf running on cable connected PC

Iperf alone could be very heavy on cpu and i think running a heavy application on your router is no real world scenario.

I think you would test your routing performance...

Why are you testing with iperf on your router and not with two cable connected pcs through nat?