The reason your laptop is faster is the integrated SSD drive. I recommend using a fast SSD as your OS and cache drives. It will dramatically speed up things. The fusion drive is just a slow HDD with some limited NAND cache that's good for running smaller applications but not really for processing large volume of RAW files. I myself use a 1TB SSD drive for processing my CR2 files. My desktop system is faster than an iMac but I can't imagine an iMac with SSD would take 8 seconds! To render a RAW. Even my MacBook Air is faster than that!