external sd - High CPU load during file copy to SD card

Copying large files to micro SD card (via MTP mode or USB thetering and FTP) causes a high CPU load (around 90%) with the result of my phone rebooting from overheating.

Is this a normal behavior? Any suggestions?

My system is Cyanogenmod 11-M8 on a Galaxy S4 mini with a SanDisk Ultra microSDXC 64GB.

EDIT

Here are the last messages before reboot according to adb logcat

I/ThermalDaemon(  322): Sensor 'tsens_tz_sensor0' - alarm cleared 1 at 47.0 degC

E/MP-Decision( 1785): num online cores: 2 reqd : 1 available : 2 rq_depth:0.000000 hotplug_avg_load_dw: 29
E/MP-Decision( 1785): DOWN cpu:1 core_idx:1 Ns:1.100000 Ts:190 rq:0.000000 seq:1069.000000
E/MP-Decision( 1785): num online cores: 1 reqd : 2 available : 2 rq_depth:2.500000 hotplug_avg_load_dw: 58
E/MP-Decision( 1785): UP cpu:1 core_idx:1 Nw:1.900000 Tw:140 rq:2.500000 seq:182.000000
I/ThermalDaemon(  322): Sensor 'tsens_tz_sensor0' - alarm raised 1 at 50.0 degC
E/NetdConnector(  832): NDC Command {4878 bandwidth gettetherstats} took too long (1006ms)
D/MobileDataStateTracker(  832): default: setPolicyDataEnable(enabled=true)