Running maven with -T1C will cause maven to try to build modules in parallel, so if you have a multi-module project, at best it will build each module’s javadoc in parallel (if your dependency graph between modules allow it).
The javadoc process itself is single-threaded, so you won’t be able to use multiple cores to generate the javadoc of one single module.
However, since you have many classes (and possibly many @link doclets or similar ?), maybe the javadoc process could benefit from extended heap. Have you looked into GC activity ?
Try adding this in your configuration, see if it helps :
<additionalJOption>-J-Xms2g</additionalJOption>
<additionalJOption>-J-Xmx2g</additionalJOption>