If we had time machines, we could destroy any currency, including cryptocurrencies.

These days, nobody does CPU mining for the major altcoins and Bitcoin. That's no surprise. Now, let's say we put a time machine into the question. We buy a beefy ASIC for Bitcoin mining, with upwards of 10 TH/s. Let's take that to the beginning of the Bitcoin mainnet using a time machine, and start mining. You can do two things from here. Either turn that baby on with no restraints and make Bitcoin worthless, or turn it on in very short intervals. For the former, the value would inflate dramatically because you own all the coins. You're mining nearly every block in mind blowing speeds. The difficulty level would also skyrocket. Since nobody can get coins easily, nobody will bother, making it worthless. Obviously, Bitcoin would hardfork with a different algorithm to stop the evil mastermind (you) from destroying Bitcoin. In the event you mine until you find a block, just to hide your ASIC, you would be fine. Do this in intervals until you have a decent amount of bitcoin. Since the block reward would be so great, just bring your wallet into the future and enjoy your money. You can even buy another miner with all that bitcoin you earned and repeat. This is only assuming that Bitcoin is not worth nothing at that point.