It is possible you may need to adjust your rendering settings to accommodate your hardware ....... SkyMAXX isnt a free ride and storms will affect frames since more clouds are drawn as a result........ A few more questions, are you using HD mesh? Is this heavy drop happening in areas where you are using orthos or third party scenery? Are you setting these storms manually with high visibility? The reason Im asking these questions although your specs are decent, I wouldnt consider this MAC a powerhouse, some sacrifices will need to happen in order to tweak performance, again there are no free rides...... EDIT: I cannot see your rendering options to make any suggestions BTW......But water reflections to low, world draw distance to medium and textures to high are a good start.....