Macbook is suddenly very slow

Question

Suddenly my MacBook Pro is very slow.  Spinning beach ball of death appears across platforms.  Might have a virus or malware.  What can I run to detect what's up?  Any suggestions?

Answer 1

You may be able to improve the speed with some software work but it could also be your disk or SSD is corrupt or failing.
Backup your data immediately:
Before proceeding you should make sure you have a current backup.  If it has been getting slower over time your disk could be a failing slowly.  SATA drives have two retry modes: short and long.  They can continue operating as read errors are increasing.  The long retry cycle is very long and can cause significant performance degradation.  If it has started long retries then it is likely to get worse and eventually fail.  SSDs also have a failure mode where the fail slowly.
Time Machine Basics: http://support.apple.com/kb/ht1427
Most commonly used backup methods: 
https://discussions.apple.com/docs/DOC-3045
Methodology to protect your data.  Backups vs. Archives.  Long-term data protection:
https://discussions.apple.com/docs/DOC-6031
Try resetting PRAM and SMC:
http://support.apple.com/kb/PH4405
http://support.apple.com/kb/HT3964
Is it faster now?  If not, proceed.
Restart in Safe Mode:
http://support.apple.com/kb/HT1564?viewlocale=en_US
Reboot in normal node.  (This boot may be slower than normal.)
Is it faster now?  If not, proceed.
Restart in Safe Mode again:
Uninstall all unnecessary login items:
 > User&Groups > Login items. > Unlock the padlock. > Remove them with the minus -
Also uninstall internet plug-ins in your browsers.
Reboot in normal node.  (This boot may be slower than normal.)
Is it faster now?  If not, proceed.
Spotlight:  If Spotlight is continually running, re-index Spotlight:
  http://support.apple.com/kb/ht2409
Low disk space:
If your disk has less than 10% free space or less than 15 GB free it can slow down your Mac.  For information on how to clear disk space see https://discussions.apple.com/thread/5634300?tstart=30 .
Disk Diagnosis and Repair; Runaway Processes:
See Kappy’s suggestions "Things You Can Do To Resolve Slow Downs” at https://discussions.apple.com/thread/5635990?tstart=0
- Directions on how to detect a failing drive
- How to repair a filesystem on a working drive.
- How to check for runaway processes hogging the processor time.
Also see Activity Monitor – Monitor Performance Problems  
http://www.scsc-online.com/Activity%20Monitor.html
Additional suggestions in "Why is my computer slow?” are particularly good.
https://discussions.apple.com/docs/DOC-3521
In the "Something is wrong with your machine” section.
- Corrupted Caches
Using Onyx to clear the caches.
- Step by Step to Fix your Mac:
https://discussions.apple.com/docs/DOC-3353
          (I learned a lot reading this document.  Good stuff.)
- A clean install.
After you have backed up your system twice to two different disk drives.
  Preferably with one of the backups being a clone.
  Time Machine restores are complex and can sometimes fail.
  You don’t want to risk all your data on one backup as you wipe out the data on your Mac.
As part of this clean install procedure for disks, erase the disk with Security Option Zero
  that writes zeros to all sectors and remaps bad sectors as it does so.
  That is a time consuming operation, but worth it.
See https://discussions.apple.com/message/24057867#24057867
If you have an SSD, to not write a pass of zeros to it.
Additional references:
http://www.macworld.com/article/2026650/mac-troubleshooting-what-to-do-when-your -computer-is-too-slow.html
and Mac Performance Guide: http://www.thesafemac.com/mpg/
and Why your Mac runs slower than it should
http://www.chriswrites.com/2009/04/17-reasons-why-your-mac-is-so-unbelievably-sl ow/
Hardware replacement or upgrade?
Hopefully by now your Mac is back to its normal speed.  If a hardware repair is indicated take it to an Genius bar at an Apple store for a free diagnosis and estimate of repairs.  If it is under warranty or AppleCare they will repair it for free.  If not, you can compare their estimate with DIY repairs.
Genius reservation http://www.apple.com/retail/geniusbar/ .
or find an Apple Authorized Service Provider https://locate.apple.com/country
You can speed it up further by upgrading some hardware. (Do not upgrade the hardware if it is still covered by the warranty or AppleCare.)  For the following memory, disk or SSD replacements and upgrades check out OWC http://eshop.macsales.com/, Crucial: http://www.crucial.com/ and Kingston http://www.kingston.com/us/.  You will need to know your Mac the year and season your Mac was first sold and model number which can be found here:
 > About This Mac > More Info… > System Report > Model Identifier:
If your Mac has 2 or 4 GB of RAM, consider an upgrade to 8 GB (if the memory is upgradeable).  An 8 GB memory upgrade is only $100.  When I upgraded a MacBook from 2 GB o 8 GB it booted thee times faster.
If your Mac has a disk and it won’t repair properly, is low on disk space, or you just want a considerably faster disk, check out an upgrade to an SSD-Hybrid disk. Google "Seagate 1TB Solid State Hybrid Drive SATA 6Gbps 64MB Cache 2.5-Inch" or consider an SSD (120 to 480 GB).
OWC has SSD replacements and upgrades for MacBook Airs and Retina MacBook Pros.
Good luck.  May the software fixes do the trick.