The WubiGuide shows how to uninstall Wubi both automatically and manually.
Installing Wubi is not likely to have any effect on your computer speed (in Windows) unless you are very low on space. The main part of a Wubi install is the virtual disk which is the file
\ubuntu\disks\root.disk (on the 'drive'/partition you installed to). By removing this and other Wubi files (through the normal uninstall process, or manually if required) this will free up the space and restore your computer back to the original state prior to install.
I recommend running
chkdsk and defragmenting your Windows drive as well. After this is done, you can make sure that you have recovered the space. In rare cases the root.disk may be corrupted and recovered by Windows to a hidden
\found.000 folder. After running chkdsk you can check for these and remove them if you don't think the space has been fully recovered. Note: the
\found.000 folder is hidden and protected by Windows.