A time-saver is a thing, a way of doing something, or sometimes a person, that lets you finish a job in less time than it would normally take. It is a noun, and it is almost always countable: a real time-saver, a great time-saver, one of the biggest time-savers in the kitchen. You use it about the thing itself, not about the saving: the rice cooker is a time-saver; the rice cooker saves time. Notice that it is usually praise. When someone calls something a time-saver, they are recommending it — 'The new form is a real time-saver' means 'use it, it's better'. It is a very common word in advertising, in office talk, and in cooking. Big things get called time-savers (a machine, a computer system) but so do very small tricks (cutting the vegetables the night before). Two more things worth knowing, though. First, the spelling wanders: you will see time-saver, time saver and timesaver, and all three are accepted; the hyphen is the safest for a student. Second, people often say it about something that only feels faster. If your friend says the app is a huge time-saver, that is their opinion, not a measured fact — so a sentence like 'It's supposed to be a time-saver' carries a small doubt with it.