Hi,
The conclusion is that the main bottleneck to boot QEMU is fw_cfg.
https://www.kraxel.org/cgit/qemu/log/?h=rebase/fw-cfg-dma-wip
Some experimental (and untested) bits implementing a dma interface for fw_cfg (also some unrelated fw_cfg stuff).
You might want try wire that up for x86 and see how it speeds up things ...
cheers, Gerd