My hAP use very hight CPU when enable Queue Tree

Hello every one, hopefull someone help me with this problem.

I have 1 hAP and an Internet Line (PPPoE, 20Mbps).

I configed Internet is ok and I made an hotspot on hAP for about 5-7 pc.

When I monitor the router with Winbox, I see when someone download at fullspeed (20Mbps), my router with have 70% CPU Load.

And after that I made the Mangle Firewall Rule to Label the packets send to the hotspot clients, my rule is:

chain=postrouting action=mark-packet new-packet-mark=down-packet passthrough=no dst-address=192.168.88.0/24 out-interface=ether2-lan log=no log-prefix=""

And I made the new PCQ Queue Type:

name="download-pcq" kind=pcq pcq-rate=5M pcq-limit=100 pcq-classifier=dst-address pcq-total-limit=20k pcq-burst-rate=0 pcq-burst-threshold=0 pcq-burst-time=10s pcq-src-address-mask=32 pcq-dst-address-mask=32 pcq-src-address6-mask=64 pcq-dst-address6-mask=64

And I made the Queue Tree:

 
 0   name="global-in" parent=global packet-mark="" limit-at=0 queue=default priority=8 max-limit=18M burst-limit=0 burst-threshold=0 burst-time=0s 

 1   name="hotspot01" parent=global-in packet-mark=down-packet limit-at=0 queue=download-pcq priority=8 max-limit=0 burst-limit=0 burst-threshold=0 burst-time=0s

And after that when two pc download at 5 Mbps (use 10Mbps total), my router work at 80-100% CPU Load.

I have tried use the simple queue but the result is look like the same.

I wonder is it because I have made the wrong-configs some where!? Thank you so much if you could show me where I have worng.