systemd-analyze plot > boottimes.svg

Open the SVG and have a look at what's happening during boot.

journalctl -b will give you some more info too. If you're using grub to boot (probably in /boot/grub/grub.cfg), you can change the loglevel and add the udev option to get a bunch more info. Helped me with a random issue recently. Here's mine for an example:

### BEGIN /etc/grub.d/10_linux ###
menuentry 'Arch Linux' --class arch --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-simple-a96b3354-70dd-45ed-8c6c-95171e9f1e82' {
	load_video
	set gfxpayload=keep
	insmod gzio
	insmod part_gpt
	insmod ext2
	search --no-floppy --fs-uuid --set=root a96b3354-70dd-45ed-8c6c-95171e9f1e82
	echo	'Loading Linux linux ...'
	linux	/boot/vmlinuz-linux root=UUID=a96b3354-70dd-45ed-8c6c-95171e9f1e82 rw  loglevel=3 udev.log-priority=debug 
	echo	'Loading initial ramdisk ...'
	initrd	/boot/amd-ucode.img /boot/initramfs-linux.img
}