How to use benchmarks to cut through marketing hype
But if I may rant for a moment, I want to call your attention to two oft-touted remedies for slow Mac performance that usually have so little effect as not to be worth the effort: freeing up disk space and defragmenting hard drives. Thats not to say these procedures are pointless or that they never help, but as with repairing permissions, their curative properties are greatly overestimated.
Fortunately, you can prove (or disprove) the effectiveness of such remedies using science! Benchmarking utilities can provide you with cold, hard, numerical factsif you take the time to use them properly.
Space invaders
Lots of utilities can locate and delete duplicate or excessively large files, old caches and logs, unwanted apps (and their supporting files scattered all over the place), unused fonts, and countless other types of data that may be cluttering up your disk. Im all for tidiness and saving disk space, and I appreciate the time and effort such apps save me.
But what bugs me about much of this software is the claim, repeated endlessly in ads and marketing copy, that deleting all this digital detritus will speed up your Mac tremendously. The implication is that theres a direct correlation between performance and disk space used.
Theres a kernel of truth in this claim. The true part is that OS X needs some breathing room to store things like virtual memory swap files; temporary files used when installing software; RAM images created when you put your laptop to sleep; and scratch files for audio-, video-, and photo-editing apps. If you run out of breathing roomwhich happens only when your disk is quite close to being fullOS X will indeed slow down, sometimes to the point of being unusable. Free up enough space, and performance should return to normal. (The need for breathing room is as true for solid-state storage as it is for hard disks, although SSDs should exhibit less pronounced speed reductions as you approach maximum capacity.)
Exactly how much free space you need to prevent performance degradation depends on quite a few variables. As a rough rule of thumb, I recommend 4 GB plus the amount of physical RAM you have installed as a reasonable minimum. But notice that this figure is independent of the size of your disk. In other words, if you have a MacBook Pro with 16GB of RAM and a 1TB disk with 990GB occupied, youre in the danger zone. But put the same files on a 2TB disk and you have loads of breathing room. In the first instance, pruning 100GB of unneeded files might have a miraculous effect on speed, while in the second, you probably wont notice any improvement at all.
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.
But before you jump in, remember that were trying to be scientific, so you must take steps to ensure that your measurements are valid. Heres what I recommend:
Of course, the fact that a benchmark number goes up significantly doesnt mean your Mac will necessarily feel faster or make you more productive. But if the numbers dont move significantly, youll know whatever you changed doesnt affect its performance, and you can save time and effort by not worrying about that thing in the future.