I had Ubuntu 18.04.3 dual boot with Windows 10 (Windows is on 2nd drive) and never had a issue with slow booting time. After installing Kali Linux on the same drive where Ubuntu is installed, Ubuntu boot time increased about 1 min, before it was 10 or 20 sec. Below is the drive info on which both OS are installed
Kali Linux: /dev/sda3 Kali linux-swap: /dev/sda4 Ubuntu: /dev/sda6 Ubuntu linux-swap: /dev/sda5
What should I do to fix the boot time?