There are some patches to improve HEVC performance but they are already included. Only parts of HEVC can be accelerated on the Raspberry Pi using the QPUs.
You could try a modest overclock; but if you want to play HEVC or HEVC 10-bit content reliably, you should consider the Vero 4K +.
The green squares appearing may be an issue with Kodi v18. As this is still experimental, it should be fixed shortly.
Sam