My MacBook Pro has been acting very slow for a while now and I'm not sure what to do...could it have a virus?

Ways to help make a slow Mac faster

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Slow Mac Performance? This Article Solves It!

Fix slow start-ups in OS X | MacFixIt - CNET Reviews

6 Easy Tips to Speed Up OS X Yosemite on Your Mac

Avoid using any third-party software that claims to clean up your computer. Usually this software does more bad than good. Furthermore, you don't need it. Note that all computers will become slower over time even under normal use. Experienced users typically erase the hard drive and do a clean install from scratch at least once a year or whenever installing a major OS upgrade. Of course doing so also means you must maintain regular and multiple backups.

Follow these steps:

- Shut down your Mac

- Wait until your computer turns off and after that press the Power button

- Right after you hear the startup tone, press and hold the Shift key

- Release the Shift key when you see a grey Apple sign and the progress bar below this sign

- Once you see Desktop, start a Disk Utility scan to detect and repair file system errors (don't forget to choose your main hard drive)

- Click on Verify Disc and then, if asked to fix problems, on Repair Disk

- After this, click on Verify Disk Permissions and then on Repair Disk Permissions

- After the process is finished, shut down your Mac and turn it back on after about 30 seconds

Apart from that, take a look at this Apple article and follow the instructions on Resetting NVRAM shown there:How to Reset NVRAM on your Mac - Apple Support

Hope this helps!

Post an Etrecheck report. It may offer some clues to your problem:

http://www.etresoft.com/etrecheck

Ciao.