The Viltrox EF-M2 just received a firmware update that improved AF speed. You can download a copy here.
EF-M2 Version 3.0
Update Date: 2018-05-22
Update as follows:
1. Improved the auto focus speed in Olympus E-M1 and E-M1 II.
2. Improved the compatibility for BlackMagic Design.
3. Improved the stabilization for GH4. In picture mode, please turn off the lens IS when camera idle.
Attention: Please dismount the device when you upgrade.