/usr/tmpDSK vs physical partition
It was perhaps created with a cpanel script rather than by the distro. It has security advantages.
I would be surprised if there was a measurable difference in speed. The small overhead of the loopback layer is going to be lost in the variation noise of any measurements you might make. It might even be slightly faster than a separate partition for /tmp, because the head seek times will be reduced.
If you are using some application that is speed bound by file access to /tmp (unlikely), then you would look at getting more memory (to provide more file caching space) or even create /tmp as a ram based filesystem.