Greetings yurinova,
To understand your situation I need some clarification. cpu-z shows the mem speed at half the actual because it's giving you information on a per stick basis. The bios will show you both the memory multiplier and the actual memory speed. For instance, if the bios say it's actually running at 1600 the cup-z wil show 800 per stick. Too, if you're running 32bit os it will only see 4gigs.
Now as to the system slow down. You'll need to be more specific. Apps are slow loading? Screen is slow responding to keyboard or mouse? Computational orperations are slowing down (ie reloading and computing a large spread sheet)? In general, here are the steps I go thru:
1. Check disk and defrag - don't know how that works with an ssd if its the os disk. Which gets me the part b: what are the relevent methods for optimizing an os ssd? Hopefully, someone here will jump in on this one.
2. Check for services being run. The first I normally go after are: shadow copy, indexing and firewall/virus protection. Too, software can add services which run all the time unnecessarily. So ... do graphics or sound devices leave extra services running? And, every sw wants to leave an update daemon running. You can try to disable some services and see if this helps.
3. Lastly check disk and defrag your data disks.
Short of all this ... I'm thinking that the memory is ok but that windoze is being its typical self. <BG>
Be well,
Kim