In general, there are two ways to speed up a Mac: upgrade hardware, optimize macOS system. Just do a quick search in Apple Support Community, you’ll find tons of optimization tips - though many of which turned out to be a little unhelpful. Reasons behind a slow Mac vary, so are the ways to tune it up. Over time hard drives accumulate all kinds of things that slow down macOS, especially latest 10.13 High Sierra which requires more available storage and memory.
Is your Mac machine running a little slowly? Does it hand up more often? MacBook, iMac, it doesn’t matter.
