Not sure, the code has been rotting for a while, and various other systems that it depends on have been modified. Maybe a lock was introduced somewhere in the render related code. A number of other features, such as audio and packaging seem to have new issues as well. I’m going through everything with a fine-tooth comb in the next couple weeks. Not everything will be finished for 4.12 as we’re branching in a few days, but I hope to address at least the performance and packaging issues.