That is essential. The tasks are done in my implementation in a different thread. Also essential: use as much buffers you can afford, at least 4 or so.
And yes, the old way of doing things worked better.
I use a RP4 for scanning, but I am using RP3 as well with the HQ or GS sensor. I am away this weekend from hardware and software, but I will report some framerates in a few days. I think request + jpeg-encode of 3.5 fps for 2k images can be speeded up a little.
@dgalland First update. Have a look at this post, especially where I talk about increasing CMA-memory for getting stuff to run on a RP3, and the configuration option to turn on the queue of libcamera. Both options help in increasing fps.