How do I speed up iMac and Safari?

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.

Then download and install theOS X Yosemite 10.10.3 Combo Update.

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.

Then download and install theOS X Yosemite 10.10.3 Combo Update.

Note: You will need an active Internet connection. I suggest using Ethernet if possible

because it is three times faster than wireless.

How much RAM is installed? How full is your hard drive?

Running anti-virus or so-called "cleaing/optimizing/tune-up" software will slow your computer.

Adware picked up by downloading form unscrupulous download hosting sites or torrent site can slow your computer. Please see this Apple article:

Remove unwanted adware that displays pop-up ads and graphics on your Mac - Apple Support

I've been using this OmniDiskSweeper to see where all my space is going! Great to check whats eating all your memory.

Please describe the problem in as much relevant detail as possible. The better your description, the better the chance of a solution.

For example, if the computer is slow, which specific actions are slow? Is it slow all the time, or only sometimes? What changes did you make, if any, just before it became slow? Have you seen any alerts or error messages? Have you done anything to try to fix it? Most importantly, do you have a current backup of all data? If the answer to the last question is "no," back up now. Ask if you need guidance. Do nothing else until you have a backup.