How to quickly assemble h264 mpg video from two clips?

Question

Video

Often and a lot we render video by gluing two clips together. The first, long, is a static picture. The second, short one is intensive animation. The output is one mpg / H264 for YouTube.
The first one is rendered each time from a new image. The second clip is always the same, prepared in advance.
How can you do this as quickly as possible with a high quality output file?
Suitable for the first long part -preset veryfast -tune stillimage. The second one was prepared with -preset veryslow -tune animation.
We tried two , as it seems to me, inefficient options:

1. render long static with rich animation parameters of the second clip, and glue them together ffmpeg -f concat -i list.txt
   
2. read a long clip with parameters for statics (faster), but then glue it together with the concat filter, which actually recalculates the entire long part.
   

Is there any way to render the static part super-fast, and still skeletonize it with the animated part as fast as concat file1|file2?