After installing the new Intel HD 4600 Graphics Driver version 15.33.18.64.3496 the DPC load is down to almost zero and the clock frequency to around 40% (idle).
In detail:
"Intel Management Engine Interface" and "Intel 8 Series / C220 Series USB EHCI #2 - 8C2D" deinstalled. But they reinstalled automatically. High DPC load unchanged.
Deinstallation NVIDIA Geforce GT740 M driver.
Deinstallation Intel HD Graphics 4600 driver.
Installation Intel HD 4600 Graphics Driver Version 15.33.18.64.3496.
Success: DPC load after booting with external monitor is down to almost zero, clock frequency at 40 %.
Installation Nvidia Graphics Driver Version 335.23
All is fine, "Intel Management Engine Interface" is active.
Thank you for your help, Sylvia !