The more controls you add to forms, the more processing there might be. So it's possible. However, there might be something more specific to section templates, although we are not specifically aware of such a problem.
Did you check memory usage? Can you increase the JVM heap size?
Note that we have performance items as part of our roadmap:
https://github.com/orbeon/orbeon-forms/wiki/Orbeon-Forms-Roadmap#performance-Erik