Yes, the lack of a GPU that can help may well be an issue - CPU is much, much slower so the default two hour render time may elapse before the render is much converged (pixels have reached a fairly stable value). Sine even the well-lit surfaces are noisy I think it isn't immediately going to help to add more light, though you may later find you need to add some weak, soft lights to slightly fill the shadowed areas and thena djust the Tonemapping to balance. The main thing to try is extending the rendertime, which is in seconds, or simply disable it by setting the Maximum Time to -1.