You can disable individual scopes in the Dash Plugins section of Applications Lens. Ubuntu 13.10 brought the Smart Scopes feature with lots of feature packed scopes. This may make Unity slow and resource hungry. But it brought a good thing too: you can now selectively disable individual Scopes from the Dash itself.

Previously, scopes could be disabled only by unchecking the corresponding schema entries using dconf or gsettings. This is not needed now as it can be done from Dash itself. As a more aggressive measure, you can uninstall the unwanted Scopes and Lens. These are packages named like unity-scope- or unity-lens-. For example, I usually remove unity-scope-musicstores and **unity-scope-video-remote* scopes and unity-lens-music and unity-lens-video. You may remove scopes and lens which use online search or the ones you don't need.

As suggested by Sushantp606 in this Answer you may disable Online Searches in Unity Dash. To do so, goto System Settings ▸ Privacy ▸ Search Results ▸ When searching in the Dash: Include online search results and turn it off.

Turning off Record Activity will affect all searches, both online and offline. Don't turn it off, unless you don't mind Dash not displaying recently used Apps, Files, Music, etc.

If you're experiencing slowness during the first search on Dash after startup, it's the startup delay of Dash. It'll speed up after a while. The second or subsequent searches will be fast. You may use preload or similar daemons to decrease startup delay.

Disabling Blur or changing to Static Blur can increase Dash performance,but won't do anything in particular to increase Search efficiency. For suggestions to improve Unity Dash performance on the whole,and not just with respect to Online Searches refer to How to speed up the Dash response? and How can I speed up the dash?