Assumption: Hugin was compiled with OpenMP and/or GPU support,
which may be checked (from the Terminal) by:
$ enblend -v -V | grep -E 'Extra.*(MP|GPU|cache)'
Extra feature: image cache: yes
Extra feature: GPU acceleration: yes
Extra feature: OpenMP: no
(for GPU-only version)
and
$ enblend-mp -v -V | grep -E 'Extra.*(MP|GPU|cache)'
Extra feature: image cache: no
Extra feature: GPU acceleration: yes
Extra feature: OpenMP: yes
(for GPU and OpenMP [SMP] version)
If neither 'Extra feature' says 'yes' then that version will only run on a single CPU thread.
Hugin does not come with this enabled by default nor detect it automatically, at least not in version 2014.0.0.5da69bc383dd - as provided in the 'Software Center' on Ubuntu (14.04).
To enable: Open/Run Hugin, select Preferences in the File menu... Click /Programs\ in the dialog that opens.
Change the enblend executable to be enblend-mp, and the same with enfuse: enfuse-mp.
If you have the hardware you may also add --gpu (dash-dash+'gpu') to "Default arguments" for enblend (there is no such argument for enfuse). Which may be enabled also without ...-mp above.
You might wish to try [v] Use GPU for remapping (EXPERIMENTAL) - but as it is "EXPERIMENTAL" it might not work correctly, depending on the content of your project.
Note that OpenMP is mutually exclusive to the use of image cache (the executable above says 'Image cache: no', 'Image cache' cannot be used).