You don't even propose any arguments why your question should not be closed, but here it goes:
Your question is too broad. You ask (at least) three question at once, only some of which really can be answered and are based on an actual problem rather then speculation.
"So how does disk encryption usually work?" would be an okay question if it showed a little research effort. It might be interesting for our Stack Exchange site that focuses on IT Security, but only if it had a little more substance to begin with. Read up on common disk encryption algorithms and implementations, then see where you can go from that.
"How can I test this with my own machine? I want to calculate the guesses per second my machine can make." – could be an interesting question, but should probably be a separate one. Give us more details about your machine.
"How many guesses per second are possible against an encrypted disk with the fastest PC ever so far?" – the fastest PC ever so far? Do you smell the not constructive part here? What is the fastest PC? What is a PC even? What disk? What disk encryption?