When kdenlive generates the XML file it passes to melt for rendering, it gives the `<consumer>` element the attribute `real_time="-1"`. As a result, melt will only use one thread to do frame rendering. The speed of rendering (especially when visual effects are involved) can be improved tremendously by setting this to the negative of the number of CPU cores instead. In my case, that would be `real_time="-8"`. The negative ensures no frame drops. Since my system has no GPU, my experiments here apply only to CPU-based rendering. Depending on how GPUs are utilized, it may or may not be beneficial to do this only if no GPU is involved.
The real_time parameter can be defined and passed to MLT by using the "Parallel processing" option in Kdenlive render dialog