There are a few things you can do to make Kali Linux faster in virtualbox. 

Allocate more RAM to Kali Linux in virtualbox settings.

Use a faster virtual disk.

Increase the CPU priority for your VM.

Turn off unneeded services.

Use a lightweight desktop environment.

Use a faster networking interface.

A higher screen resolution will use more resources and slow down your VM. Try a lower resolution for better performance.

Use the Kali Linux minimal install. 

SWIPE UP TO LEARN ETHICAL HACKING FOR FREE

JOIN TELEGRAM