if you enabled l2-learning global-mode switching, inter-vlan traffic is handled by the switching chip. Only traffic destined outside the vlan will be send to the CPU.
Where do you see the CPU load of 71%? via junos-commands or via top? Please provide output of the relevant commands.
when looking at my own SRX300 I have the following scenario. Please have in mind that the flowd_octeon_hm process is intended to use almost all capacity on CPU1.
jh@fw> show chassis routing-engine
Routing Engine status:
Temperature 60 degrees C / 140 degrees F
CPU temperature 77 degrees C / 170 degrees F
Total memory 4096 MB Max 2499 MB used ( 61 percent)
Control plane memory 2624 MB Max 2125 MB used ( 81 percent)
Data plane memory 1472 MB Max 368 MB used ( 25 percent)
5 sec CPU utilization:
User 10 percent
Background 0 percent
Kernel 2 percent
Interrupt 0 percent
Idle 88 percent
Model RE-SRX300
Serial ID XXXXXXXXXXXXX
Start time 2016-09-20 17:12:21 CEST
Uptime 20 days, 23 hours, 22 minutes, 30 seconds
Last reboot reason 0x200:normal shutdown
Load averages: 1 minute 5 minute 15 minute
0.12 0.23 0.20
jh@fw> show system processes summary
last pid: 79590; load averages: 0.04, 0.19, 0.18 up 20+23:24:01 16:35:52
162 processes: 17 running, 133 sleeping, 12 waiting
Mem: 338M Active, 151M Inact, 1541M Wired, 432M Cache, 112M Buf, 1517M Free
Swap:
PID USERNAME THR PRI NICE SIZE RES STATE C TIME WCPU COMMAND
1762 root 5 139 0 1552M 470M CPU1 1 564.6H 92.48% flowd_octeon_hm
21 root 1 171 52 0K 16K RUN 0 404.8H 82.67% idle: cpu0