[Haze_hellivo]

Most likely its the splitter. try pluggling each fan alone and see if rpms rise. Fan controlers have a certain wattage available per channel (Ex: 13w, 10w,18w) if you link two fans in the same channel and they consume more watts then channel as to offer they run at lower rpms.

 

[Haze_hellivo]

For those fans, get a fan controller with 4 or 6 channels 18w/30w per channel.

Link: http://www.performance-pcs.com/catalog/index.php?main_page=product_info&cPath=36_48&products_id=30058