Why is nmap that slow on my system (ubuntu 14.04)?

I just started to learn something about portscanning with nmap. when i try to run an a nullscan or any other type on a virtual machine (win7 no firewall) the scan needs a lot of time to perform:

sebi@sebi-UX31A:~$ sudo nmap -sN 192.168.1.109

Starting Nmap 6.40 ( http://nmap.org ) at 2015-01-01 22:07 CET
Nmap scan report for win7-PC (192.168.1.109)
Host is up (0.00054s latency).
All 1000 scanned ports on win7-PC (192.168.1.109) are closed
MAC Address: 08:00:27:4D:CF:E8 (Cadmus Computer Systems)

Nmap done: 1 IP address (1 host up) scanned in 45.53 seconds

Unfortunately I have no idea how to solve the problem. I hope someone can help me.