Sent out today were the Xen patches for Linux 5.16. Besides having some code lcean-ups, para-virtualized interrupt masking made simpler, Xen "pciback" driver support for Arm, and other smaller enhancements, there is also work to speed-up guest booting. In particular, the booting up of Xen PV (para-virtualized) guests should be much quicker with Linux 5.16 and beyond.
Jan Beulich of SUSE wrote in the prior patch series when first proposing the Xen PV guest boot optimizations, " The observed (by the human eye) performance difference of early boot between native and PV-on-Xen was just too large to not look into. As it turns out, gaining performance back wasn't all that difficult. While the series (re)introduces a small number of PTWR emulations on the boot path (from phys_pte_init()), there has been a much larger number of them post-boot. Hence I think if this was of concern, the post-boot instances would want eliminating first. "
More details via the Xen pull request for the Linux 5.16 kernel merge window that is ending this weekend.