start up slow on os x yosemite

It would help us to help you if we could have some more technical info about your iMac.

If you like, please go ahead and download, install and run Etrecheck.

Etrecheck was developed as a simple Mac diagnostic report tool by a regular Apple Support forum user and technical support contributor named Etresoft. Etrecheck is a small, unobstrusive app that compiles a static snapshot of your entire Mac hardware system and installed software.

This is a free app that has been honestly created to provided help in diagnosing issues with Macs running the new OS X versions.

It is not malware and can be safely downloaded and installed onto your Mac.

http://www.etresoft.com/etrecheck

Copy/paste and post its report here in another reply thread so that we have a complete profile of your Mac's hardware and installed software so we can all help with your Mac performance issues.

Thank You.

EtreCheck version: 2.0.11 (98)

Report generated 16 November 2014 9:19:32 am NZDT

Hardware Information:ℹ️

MacBook Air (11-inch, Mid 2011) (Verified)

MacBook Air - model: MacBookAir4,1

1 1.6 GHz Intel Core i5 CPU: 2-core

4 GB RAM Not upgradeable

BANK 0/DIMM0

2 GB DDR3 1333 MHz ok

BANK 1/DIMM0

2 GB DDR3 1333 MHz ok

Bluetooth: Old - Handoff/Airdrop2 not supported

Wireless: en0: 802.11 a/b/g/n

Video Information:ℹ️

Intel HD Graphics 3000 - VRAM: 384 MB

Color LCD 1366 x 768

System Software:ℹ️

OS X 10.10 (14A389) - Uptime: 5 days 21:8:6

Disk Information:ℹ️

APPLE SSD SM128C disk0 : (121.33 GB)

S.M.A.R.T. Status: Verified

EFI (disk0s1) <not mounted> : 210 MB

Recovery HD (disk0s3) <not mounted> [Recovery]: 650 MB

Macintosh HD (disk1) / [Startup]: 120.10 GB (101.44 GB free)

Core Storage: disk0s2 120.47 GB Online

USB Information:ℹ️

Apple Inc. BRCM20702 Hub

Apple Inc. Bluetooth USB Host Controller

Apple Inc. Apple Internal Keyboard / Trackpad

Apple Inc. FaceTime Camera (Built-in)

Thunderbolt Information:ℹ️

Apple Inc. thunderbolt_bus

Gatekeeper:ℹ️

Mac App Store and identified developers

Problem System Launch Daemons:ℹ️

[failed] com.apple.ctkd.plist

Launch Agents:ℹ️

[loaded] com.oracle.java.Java-Updater.plist Support

Launch Daemons:ℹ️

[loaded] com.adobe.fpsaud.plist Support

[loaded] com.oracle.java.Helper-Tool.plist Support

User Launch Agents:ℹ️

[loaded] com.google.keystone.agent.plist Support

User Login Items:ℹ️

iTunesHelper Application (/Applications/iTunes.app/Contents/MacOS/iTunesHelper.app)

Internet Plug-ins:ℹ️

FlashPlayer-10.6: Version: 15.0.0.223 - SDK 10.6 Support

Flash Player: Version: 15.0.0.223 - SDK 10.6 Support

QuickTime Plugin: Version: 7.7.3

JavaAppletPlugin: Version: Java 8 Update 25 Check version

Default Browser: Version: 600 - SDK 10.10

3rd Party Preference Panes:ℹ️

Flash Player Support

Java Support

Time Machine:ℹ️

Time Machine not configured!

Top Processes by CPU:ℹ️

7% WindowServer

2% launchd

1% Finder

1% discoveryd

1% fontd

Top Processes by Memory:ℹ️

176 MB com.apple.WebKit.WebContent

77 MB Safari

69 MB discoveryd

43 MB WindowServer

34 MB mds_stores

Virtual Memory Information:ℹ️

1.53 GB Free RAM

1.20 GB Active RAM

287 MB Inactive RAM

1.07 GB Wired RAM

7.18 GB Page-ins

397 KB Page-outs

There is not you can do.

Your MBA is not really loaded up with much and it is maxed out at 4 GBs of RAM which is really the real world "working minimum" of RAM for newer OS X versions.

Try and PRAM and SMC reset on your MacBook Air.

PRAM reset

1. Shut down your Mac.
2. Locate the following keys on the keyboard: Option, Command (⌘), P, and R. You will need to hold these keys down simultaneously in step 4.
3. Turn on your Mac.
4. Immediately press and hold the Option-Command-P-R keys. You must press this key combination before the gray screen appears.Continue holding the keys down until your Mac restarts, and you hear the startup sound for the second time.
5. Release the keys.

Resetting PRAM may change some system settings and preferences. Use System Preferences to restore your settings.

SMC Reset

Resetting the SMC on portables with a battery you should not remove on your own

Note: Portable computers that have a battery you should not remove on your own include MacBook Pro (Early 2009) and later, all models of MacBook Air, and MacBook (Late 2009).

1. Shut down the computer.
2. Plug in the MagSafe power adapter to a power source, connecting it to the Mac if its not already connected.
3. On the built-in keyboard, press the (left side) Shift-Control-Option keys and the power button at the same time.
4. Release all the keys and the power button at the same time.
5. Press the power button to turn on the computer.
   Note: The LED on the MagSafe power adapter may change states or temporarily turn off when you reset the SMC.