tl;dr

• Edit /etc/fstab. You need to comment out the lines that mount swap partitions. Ideally you should do this before deleing the swap partition
• Edit /etc/initramfs-tools/conf.d/resume (with sudo) and comment out the swap partition
• Edit /etc/crypttab if you had an encrypted swap partition
• Edit /etc/default/grub if you had hibernation enabled for Ubuntu

• You should update the /etc/fstab first, comment out the swap entries, and then delete the swap partition.

Editing /etc/fstab afterwards didn’t really fix the slow boot issue for me.

Upon installing a virtualization package with apt i came across this warning:

W: initramfs-tools configuration sets RESUME=UUID=2854a8db-6494-41d0-9cff-974714bb0b00
W: but no matching swap device is available.

The initramfs is an cpio archive. At boot time, the kernel unpacks that archive into ram, mounts and uses it as initial root file system. From there on the mounting of the real root file system occurs in user space.

resume
      The resume hook tries to autodetect the resume partition and uses  the  first  swap
      partition   as  valid  guess.  It  is  possible  to  set  the  RESUME  variable  in
      /etc/initramfs-tools/conf.d/resume.  The boot variable noresume overrides it.

I had originally created the Swap partition (a partition and no a swapfile), because a swap partition is required to be able to hibernate an Ubuntu system..

sudo nano /etc/initramfs-tools/conf.d/resume

#RESUME=UUID=2854a8db-6494-41d0-9cff-974714bb0b00

Additionally, you can also try re-configuring the initramfs-tools, but i didn’t need to do so.

sudo dpkg-reconfgure initramfs-tools

You should also check your GRUB file for a resume entry

sudo nano /etc/default/grub

You can either updated or remove the UUID value

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash resume=UUID=ad6fb579-9cdb-4b29-8977-f9aa9ee57e6f"

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"

and then

sudo update-grub