Hello,
i'm successfully using OpenVPN client with Omega2+ using AES-128 encryption without any cpu overhead problem. I'm using the LEDE project distribution re-built after re-configuring it through the 'make menuconfig'. A short steps list follows:
Download from github (git clone) the latest LEDE project source repository
'make menuconfig' and add OpenVPN plus SSL and Crypto libraries (and some other useful stuff you need)
'make', then copy the created firmware file (.bin) found in the bin/<target> directory to a USB key root
rename the firmware bin file to 'omega2.bin'
insert the USB key into the USB interface of the Omega 2 Dock, keep pressed the reset button and then power-on the board. - from the menu choose the command '2', then wait for the firmware file being flashed. lede distribution then will boot.
edit /etc/config/openvpn with your openvpn configuration (see openvpn manuals) and copy all certificates to /etc/openvpn directory
/etc/init.d/openvpn restart and you shoud be able to use openvpn as a client.
Hope this will help!
Leo