zoopster There is some slowness introduced by the Arduino library, which could be removed by writing your own libraries that, for example, do not need to set the MUX and IO Level shifter with every pin change, or you could even get some speed increase by using system() calls instead of arduino pin functions. You can even use Fast GPIO, but what if you need more than 2 pins?
The fact remains, you simply cannot get deterministic timing from a computing system without using an interface. The DHT sensors need deterministic timing. Galileo is a computing system, not a micro processor or micro controller. No matter how fast you could get the GPIO to work, you will never get deterministic timing reliably because your process can (and WILL) get de-scheduled regularly. In fact, in linux, the more agressively you try to hog the processor, the more aggressively the process scheduler will try to stop you doing exactly that.
How much is a day or your work worth? You could spend many days trying to optimise the GPIO libraries and building code to detect that a reading wasn't made and re-trying it, or you could spend less than $5 making a small interface which would work reliably 100% of the time.
You couldn't connect a DHT sensor to a Mac or PC without an interface, so why expect to connect it to Galileo without one?