Yosemite extremely slow. Cannot stop pop up windows

Question

Yosemite extremely slow. Cannot stop pop up windows on any browsers. Considering going back to Maverick My iMack is mid 2011 21.5. I cannot find a solution to this problem

Answer 1

Helpful Links Regarding Malware Problems
If you are having an immediate problem with ads popping up see The Safe Mac » Adware Removal Guide and AdwareMedic.
Open Safari, select Preferences from the Safari menu. Click on Extensions icon in the toolbar. Disable all Extensions. If this stops your problem, then re-enable them one by one until the problem returns. Now remove that extension as it is causing the problem.
The following comes from user stevejobsfan0123. I have made minor changes to adapt to this presentation.
Fix Some Browser Pop-ups That Take Over Safari.
Common pop-ups include a message saying the government has seized your computer and you must pay to have it released (often called "Moneypak"), or a phony message saying that your computer has been infected, and you need to call a tech support number (sometimes claiming to be Apple) to get it resolved. First, understand that these pop-ups are not caused by a virus and your computer has not been affected. This "hijack" is limited to your web browser. Also understand that these messages are scams, so do not pay any money, call the listed number, or provide any personal information. This article will outline the solution to dismiss the pop-up.
Quit Safari
Usually, these pop-ups will not go away by either clicking "OK" or "Cancel." Furthermore, several menus in the menu bar may become disabled and show in gray, including the option to quit Safari. You will likely have to force quit Safari. To do this, press Command + option + esc, select Safari, and press Force Quit.
Relaunch Safari
If you relaunch Safari, the page will reopen. To prevent this from happening, hold down the 'Shift' key while opening Safari. This will prevent windows from the last time Safari was running from reopening.
This will not work in all cases. The shift key must be held at the right time, and in some cases, even if done correctly, the window reappears. In these circumstances, after force quitting Safari, turn off Wi-Fi or disconnect Ethernet, depending on how you connect to the Internet. Then relaunch Safari normally. It will try to reload the malicious webpage, but without a connection, it won't be able to. Navigate away from that page by entering a different URL, i.e. www.apple.com, and trying to load it. Now you can reconnect to the Internet, and the page you entered will appear rather than the malicious one.
An excellent link to read is Tom Reed's Mac Malware Guide.
Also, visit The XLab FAQs and read Detecting and avoiding malware and spyware.
See these Apple articles:
  Mac OS X Snow Leopard and malware detection
  OS X Lion- Protect your Mac from malware
  OS X Mountain Lion- Protect your Mac from malware
  OS X Mavericks- Protect your Mac from malware
  About file quarantine in OS X
If you require anti-virus protection Thomas Reed recommends using ClamXAV. (Thank you to Thomas Reed for this recommendation.)
These may be helpful in improving Yosemite's performance.
Try these in order testing your system after each to see if it's back to normal:
1. a. Resetting your Mac's PRAM and NVRAM
    b. Intel-based Macs: Resetting the System Management Controller (SMC)
2. Restart the computer in Safe Mode, then restart again, normally. If this doesn't help, then:
Boot to the Recovery HD: Restart the computer and after the chime press and hold down the COMMAND and R keys until the Utilities menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.
3. Repair the Hard Drive and Permissions: Upon startup select Disk Utility from the Utilities menu. Repair the Hard Drive and Permissions as follows.
When the recovery menu appears select Disk Utility. After DU loads select your hard drive entry (mfgr.'s ID and drive size) from the the left side list.  In the DU status area you will see an entry for the S.M.A.R.T. status of the hard drive.  If it does not say "Verified" then the hard drive is failing or failed. (SMART status is not reported on external Firewire or USB drives.) If the drive is "Verified" then select your OS X volume from the list on the left (sub-entry below the drive entry,) click on the First Aid tab, then click on the Repair Disk button. If DU reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported click on the Repair Permissions button. Wait until the operation completes, then quit DU and return to the main menu. Select Restart from the Apple menu.
4. Reinstall Yosemite: Reboot from the Recovery HD. Select Reinstall OS X from the Utilities menu, and click on the Continue button.
Note: You will need an active Internet connection. I suggest using Ethernet if possible
            because it is three times faster than wireless.
5. Reinstall Yosemite from Scratch:
Be sure you backup your files to an external drive or second internal drive because the following procedure will remove everything from the hard drive.
How to Clean Install OS X Yosemite
Note: You will need an active Internet connection. I suggest using Ethernet if possible
            because it is three times faster than wireless.