Theres also the matter of what you delete. If your Mac is running slowly because it has insufficient disk space for virtual memory swap files, then deleting a couple of big files might help a lot. But if its running slowly because a particular buggy app is out of control, then only deleting (or disabling) that app will help. If you let a utility uninstall dozens of apps, disable login items, and clear caches, that might help your speed problembut not necessarily for the reason you think.
Fragments of truth
When your Mac writes a file to a hard disk, there may not be enough contiguous space to store the whole file as a single unit. Instead, your Mac stores a piece here, a piece there, and keeps a record of where all the pieces are so that they can be reassembled when you need to open the file. This all happens transparently and almost instantly. In addition, OS X automatically defragments smaller files (under 20MB) in the background.
But conventional wisdom has it that since fragmentation only increases with time, eventually disk access will slow down because the read/write head has to physically jump around so much to reach all the pieces of each file. And for that reason, several utilities can defragment your disk, rearranging all the pieces of each file so they can be read in a single pass. Defragmentation can be extremely time-consuming, and while its happening, your Mac will definitely be much slower than usual because of the constant heavy disk access. (As a side note, I should mention that SSDs dont require defragmentation, and in fact, attempting to defragment an SSD can reduce its lifespan.)
But is defragmentation worth it? Again, it depends. All things being equal, the less free disk space you have, the greater the likelihood of fragmentation, and the greater its impact on your Macs performance. If you have a large, fast hard drive thats nowhere close to being full, it will still have some fragmentation, but the real-world performance gains from defragmenting the drive will probably be trivial.
Put it to the test
If you encounter a process that purports to speed up your Mac (whether deleting files, defragmenting, or something else), you could try it and then make a subjective assessment as to whether it helped. But a much better approach is to arm yourself with facts. You can use a benchmarking utility to measure it before and after making a change and compare the numbers.
The two most popular benchmarking tools for Macs are Spiny Softwares Xbench (free) and Primate Labs Geekbench (free for basic 32-bit benchmarks, $10 for the standard version, or $100 for the Pro version). Theyre simple to usea single click will run a predefined suite of tests and give you an overall numeric score plus individual scores for various tests.
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