I assume you downloaded the package manually from the boinc web pages.
Maybe it would be better if you use the package from your distro, if availible (what distro are you using?).
can you explain a little bit more what you mean by still using 100%?
Boinc run always with the lowest possible prio (except when running the becnhmarks).
But most of the time BOINC can you 100% of the CPU, because nothing else needs CPU.
Using a web browser e.g. does not really require the CPU, only in the case when a new page is loaded, but still only a little bit of the available CPU.