A SSHD is not as fast as a SSD, no, according to Seagate they're about 10% slower. I'm running a 2 TB SSHD as my main drive and I love it. Boot times are significantly faster than a standard HDD, and I'll deal with it being 10% slower than a SSD for 2 TB of space when an SSD at the same price might get you 120 GB. The cache is basically the same as how your RAM caches stuff for the operating system to access quickly. The HDD with a memory cache keeps commonly used info saved so it can more rapidly execute instructions.