I just installed Kali Linux (on /dev/sda5) in dual boot with Ubuntu (on /dev/sda2). However, now that I have Kali, Ubuntu takes a few minutes to launch, whereas Kali takes a few seconds, and I would like the contrary. How can I do that ?

Edit

Here is my /etc/fstab in Ubuntu :

# <file system> <mount point>   <type>  <options>       <dump>  <pass>

# / was on /dev/sda2 during installation

UUID=7d4d2f18-146c-4d56-b5f3-0dc605eeb9e0 /               ext4    errors=remount-ro 0       1

# /boot/efi was on /dev/sda1 during installation

UUID=95B2-5AED  /boot/efi       vfat    umask=0077      0       1

# /home was on /dev/sda3 during installation

UUID=69d6623e-0bcc-4cef-8b25-e46c98210d44 /home           ext4    defaults        0       2

# swap was on /dev/sda4 during installation

UUID=97913f94-1989-4c8d-b2e0-010dfb10d613 none            swap    sw              0       0

Welcome to Ask Ubuntu.

I'd suggest exploring what you changed in installing Kali. Did you create the sda5 partition? or was it there before (primarily did you reformat it? it was accessed before in your file-system table causing Ubuntu to try & mount a now non-existing partition until it's reaches timeout as your description implies to me). We can't know what changes you made in the installation of Kali, but I'd look at 'sda5' changes if I was in your position.

If you re-formatted 'sda5' you'll have changed the UUID (or unique-ID of the partition; created during format), and your Ubuntu is likely looking and waiting for the UUID of the old-partition to appear, then eventually time-out.... ie. you need to change the UUID of your old partition, to that of the newly formatted partition. You've not amended it to reflect your new layout causing the timeouts... The first place I'd look is your file-system-table (/etc/fstab)..

Don't just replace or change UUIDs without checking that the UUID's match your partition UUID's. If you sudo blkid |grep UUID you'll get a list of UUID's found on your system, use these [results] to check the UUIDs mentioned in your fstab are correct. I don't see sda5 mentioned in comments (lines which have a # at the start are comments or documentation) but I'd still check them all. Installations (ie. kali) often re-format swap; so you may have found this has changed, and this may be your issue causing slow boots.

A rebuild of initramfs is your next step; I think it has UUID's built into it (update-initramfs). If boots are still slow, next i would probably rebuild grub (update-grub but this usually uses PARTUUID, but is quick and simple to re-create).

(this answer is just copied/pasted comments with minor editing)