If you edit /etc/mkinitcpio.conf at the bottom of the file you can uncomment the compression method. lz4 is even faster than zstd which is the default, but it makes a bigger initramfs img, so you need enough free space in the partition /boot is in. It's only a small speed difference. Also it allows old kernels before 5.9 to be used, as they didn't support zstd, otherwise they give an unpack error and won't boot. Some comparisons I found: (lz4 default is 1 according to man lz4)
https://forum.endeavouros.com/t/which-compression-algorithm-to-choose-in-mkinitcpio-conf/10457/22 (https://forum.endeavouros.com/t/which-compression-algorithm-to-choose-in-mkinitcpio-conf/10457/22)
https://insanity.industries/post/pareto-optimal-compression/ (https://insanity.industries/post/pareto-optimal-compression/)
https://github.com/archlinux/mkinitcpio/pull/47 (https://github.com/archlinux/mkinitcpio/pull/47)