Slow Down or Speed Up
Sex, like music, has a tempo. Too slow and sex is boring. Too fast and sex is chaotic and overwhelming. Sex is also very individual, so what is too fast for you may be too slow for your partner. Communication is always key.
One good general rule about tempo is to slow down. Try to look at your partner's body like you've never seen it before. Explore it and find new nooks and crannies, count to a thousand, recite baseball statistics – in short do anything you can to slow down.
There are times, however, when speed is a good thing. Speed as you move over the cliff of stimulation toward inevitable release of orgasm can be a really good thing. If your partner shouts FASTER, then by all means pick up the pace. Sex toy manufacturers know that speed is a good thing. Most build multiple speed options into their toys.