WUBI runs at native speed because it is not running an OS on top of ubuntu
or external application as in a virtual machine to make it work. It is stand
alone Ubuntu that runs on the Windows file system, but internally it is all
Ubuntu. If the Windows file system goes down though it can be taken out and
if the file system is fragmented it can be slowed down. It has its own swap
space and disk space that resides in c:\ubuntu\disks\. In hosting it Windows
does nothing other than provide the space. The magic is on the Linux end
where these are seen as real hard drives and swap partitions. The only thing
that won't work is hibernation. If you hard boot Windows you can't get into
Ubuntu until you enter Widnows and then it can successfully mount Ubuntu
after you re-boot.

WUBI is not as good as a full installation of Ubuntu for a few reasons. WUBI
is containerised and cannot grow dynamically. It can't hibernate. It is
subject to Windows which is more crash prone. It requires at least 5 GBs of
available hard drie space. You can move a WUBI installation to a partition
should you decide to move onto a dual boot situation. This requires a
separate script that you would have to Google to find. It is a good
introduction and preferable to the Live CD in giving you a better user
experience.