I've just updated Xcode to version 6.3. I'm working on a project in Swift with some imported Objective C code. I also use Cocoapods.
I had the same issue with the Xcode 6.3 beta. I've seen and tried all other StackOverflow answers with no luck.
So far I've tried to:
Delete
~/Library/Developer/Xcode/DerivedDataand~/Library/Caches/com.apple.dt.Xcodewith Xcode and/or Mac restart.Uninstall Spotify.
Not use the "+" concatenation operator.
Set deployment's target to 8.1.
I should mention that Objective C code autocompletes fast and that before indexing Swift code too. Esc button for popup suggestions works with the same delay.
Setting Whole Module Optimization to Yes is a workaround in my project.
In my case, the project has 5 modules, named here A, B, C, D and E, where the dependency is diagramed A -> B -> C -> D, and each of A to D refers E. With Xcode 6.3 and 6.3.1, it is fine to compile modules E and D, but it takes longer to compile C, and much longer to compile B and A, like 10 mins. After I change the project setting, it takes less than a min to compile all the modules. It looks faster than compilation with Xcode 6.2.
References:
Try to clear project Derived Data, after update.
Window - Projects - YourProject - Derived Data - Delete...
In my case code completion is working better after update.
I used extensions on the GameScene Class and it sped everything up.
I took out three large functions from my gameScene Class, And made three separate extensions for them.
This is probably the easiest solution if none of the above work.
This issue appears to be fixed in Xcode 6.3.1 (released yesterday).
来源：https://stackoverflow.com/questions/29579572/xcode-6-3-code-completion-too-slow