Hard to answer with so little information. Could you provide details such as installed software, is the SSD full/ in good condition etc.
One option is to download and run Etrecheck, it will give diagnostic information about your Mac which might provide clues to why the Macbook Air is slow.
www.etrecheck.com
Thanks so much for this. Info from EtreCheck is below, I'm not sure how to interpret it....?
I currently have 42GB free out of 256 storage, 8GB ram. I'm using El Capitan, v. 10.11.6.
On a typical work day I have the following apps running/open: SpiderOak, Mail, MailMate, Word, OmniOutliner, Skim, 1Password, Ulysses, Evernote, Scrivener, TaskPaper, Calendar, TextExpander, Safari, Bookends, DevonThink Sorter, Google Drive, Box, Dropbox, Hazel, Avast, Default Folder X, BetterTouchTool, Yoink, Skype, Alfred.
EtreCheck
Hardware Information:
MacBook Air
MacBook Air - model: MacBookAir6,2
1 1,3 GHz Intel Core i5 CPU: 2 cores
8 GB RAM
Video Information:
Intel HD Graphics 5000 - VRAM: (null)
Audio Plug-ins:
BluetoothAudioPlugIn: Version: 4.4.6
AirPlay: Version: 2.0
AppleAVBAudio: Version: 403.4
iSightAudio: Version: 7.7.3
Startup Items:
tap - Path: /Library/StartupItems/tap
tun - Path: /Library/StartupItems/tun
System Software:
OS X 10.11.6 (15G1217) - Uptime: 10 days 21:42:22
Disk Information:
APPLE SSD SD0256F disk0 : (251 GB)
EFI (disk0s1) <not mounted>: 209,7 MB
disk0s2 (disk0s2) <not mounted>: 250,14 GB
Recovery HD (disk0s3) <not mounted>: 650 MB
USB Information:
Apple Inc. BRCM20702 Hub
Apple Inc. Bluetooth USB Host Controller
FireWire Information:
Thunderbolt Information:
Apple Inc. thunderbolt_bus
Kernel Extensions:
com.avast.PacketForwarder (2.1)
com.avast.FileShield (3.0.0)
Problem System Launch Daemons:
[failed] com.apple.logd.plist
[loaded] org.postfix.newaliases.plist
Problem System Launch Agents:
Launch Daemons:
[loaded] com.adobe.ARMDC.Communicator.plist
[loaded] com.adobe.ARMDC.SMJobBlessHelper.plist
[failed] com.adobe.fpsaud.plist
[loaded] com.avast.init.plist
[loaded] com.avast.secureline.init.plist
[loaded] com.avast.secureline.uninstall.plist
[loaded] com.avast.secureline.update.plist
[loaded] com.avast.uninstall.plist
[loaded] com.avast.update.plist
[loaded] com.bombich.ccc.plist
[loaded] com.bombich.ccchelper.plist
[loaded] com.google.keystone.daemon.plist
[loaded] com.microsoft.autoupdate.helper.plist
[loaded] com.microsoft.office.licensing.helper.plist
[loaded] com.microsoft.office.licensingV2.helper.plist
[loaded] com.sparklabs.ViscosityHelper.plist
[loaded] com.wuala.WualaFS.KextLoaderHelper.plist
[loaded] org.gpgtools.gpgmail.patch-uuid.plist
[loaded] org.macosforge.xquartz.privileged_startx.plist
Launch Agents:
[not loaded] com.adobe.AAM.Updater-1.0.plist
[failed] com.adobe.AdobeCreativeCloud.plist
[failed] com.adobe.ARMDCHelper.cc24aef4a1b90ed56a725c38014c95072f92651fb65e1bf9c8e43c37a23d420d.plist
[loaded] com.avast.secureline.update-agent.plist
[loaded] com.avast.secureline.userinit.plist
[loaded] com.avast.update-agent.plist
[loaded] com.avast.userinit.plist
[loaded] com.citrix.AuthManager_Mac.plist
[loaded] com.citrix.ReceiverHelper.plist
[loaded] com.citrix.ServiceRecords.plist
[loaded] com.google.keystone.agent.plist
[loaded] com.logitech.manager.daemon.plist
[not loaded] com.sony.ReaderDesktop.RunReaderDesktop.plist
[loaded] net.culater.SIMBL.Agent.plist
[loaded] org.gpgtools.gpgmail.enable-bundles.plist
[loaded] org.gpgtools.gpgmail.patch-uuid-user.plist
[loaded] org.gpgtools.Libmacgpg.xpc.plist
[loaded] org.gpgtools.macgpg2.fix.plist
[loaded] org.gpgtools.macgpg2.shutdown-gpg-agent.plist
[loaded] org.gpgtools.updater.plist
[loaded] org.macosforge.xquartz.startx.plist
User Launch Agents:
[loaded] com.adobe.ARM.[...].plist
[loaded] com.akamai.single-user-client.plist
[failed] com.apple.CSConfigDotMacCert-hege.hermansen@me.com-SharedServices.Agent.plist
[loaded] com.avast.home.userinit.plist
[not loaded] com.avast.secureline.home.userinit.plist
[loaded] com.dropbox.DropboxMacUpdate.agent.plist
[failed] com.google.GoogleContactSyncAgent.plist
[not loaded] com.tidal.desktop.plist
[failed] com.wondershare.mobilegodaemon.plist
User Login Items:
Box Sync
SMS+
Citations
Default Folder X Helper
DEVONthink Personal Sorter
BetterTouchTool (215)
Alfred
BetterTouchTool
AdobeResourceSynchronizer
Dropbox
HazelHelper
NoteBookHelper
SpyderUtility
readerAppHelper
SpiderOak
CCC User Agent
Google Drive
TotalFinder
AdobeResourceSynchronizer
Jottacloud
Default Folder X
SpiderOakONE
MEGAsync
AirDisplayStatusItem
Air Display
DEVONthink Sorter
Box Edit
Box Local Com Server
HazelHelper
3rd Party Preference Panes:
Air Display Preferences
Akamai NetSession Preferences
Citrix ShareFile Sync
Flash Player
GPGPreferences
Growl
Hazel
Java
Logitech Preference Manager
MacFUSE (Tuxera)
TeXDistPrefPane
Tuxera NTFS
Internet Plug-ins:
AdobeAAMDetect.plugin
AdobePDFViewer.plugin
AdobePDFViewerNPAPI.plugin
CitrixICAClientPlugIn.plugin
Default Browser.plugin
DirectorShockwave.plugin
QuickTime Plugin.plugin
Flash Player.plugin
FlashPlayer-10.6.plugin
JavaAppletPlugin.plugin
MeetingJoinPlugin.plugin
SharePointBrowserPlugin.plugin
Silverlight.plugin
User Internet Plug-ins:
Bad Fonts:
None
Time Machine:
Skip System Files: NO
Mobile backups: OFF
Auto backup: YES
Volumes being backed up:
Destinations:
Untitled [Local] (Last used)
Total size: 499,73 GB
Total number of backups: 21
Oldest backup: 2016-01-27 16:20:07 +0000
Last backup: 2016-11-15 15:34:39 +0000
Size of backup disk: Excellent
Backup size 499,73 GB > (Disk size Zero KB X 3)
Top Processes by CPU:
20% WindowServer
12% firefox
7% Safari
3% Box Sync Monitor
2% hidd
1% Calendar
1% Box Sync
1% EtreCheck
1% Google Drive
0% com.apple.WebKit.Networking
Top Processes by Memory:
934 MB Safari
590 MB firefox
164 MB Mail
131 MB suggestd
98 MB plugin-container
82 MB WindowServer
82 MB Calendar
82 MB mds_stores
74 MB Skype
57 MB com.apple.WebKit.WebContent
Virtual Memory Statistics:
77 MB Free RAM
2.29 GB Active RAM
2.15 GB Inactive RAM
1.82 GB Wired RAM
55.34 GB Page-ins
2.98 GB Page-outs