Why Gradle is so slow?

Gradle version (File -> Project Structure, Project): Gradle 2.4
Android Plugin Version (File -> Project Structure, Project):
classpath ‘com.android.tools.build:gradle:1.1.1’

SDK Version: “Android SDK Tools” Rev. (Tools -> Android -> SDK Manager) : 21
Module Compile Sdk Version (File -> Project Structure, Your Module): 21
Module Build Tools Version (File -> Project Structure, Your Module): 21.1.2

I am trying to mirrate a project from (eclipse + Ant) to (Android Stdido + Gradle),everything goes well,but copimling is so slow!!!

The project has Over 65K Methods,so i have add Multidex.The structure is as follows:
app,module A,module B,module C,module D,module E,module F.

My laptop is hp ProBook 430.
Disk: Samsung SSD 840 Pro 256G
Memory: 12.0 GB

I execute the command:
./gradlew assembleDevelopDebug --info

for completely compiling:
:***_module_main:dexDevelopDebug (Thread[Task worker Thread 3,5,main]) completed. Took 1 mins 47.907 secs.
Total time: 4 mins 16.198 secs

for incremental compiling: (i just delete one line code)
:***_module_main:dexDevelopDebug (Thread[Daemon worker Thread 4,5,main]) completed. Took 1 mins 36.453 secs.
Total time: 3 mins 0.863 secs

My problem is :

Why Gradle is so slow? Whether or not incremental compiling.
Especially execute the task dex…

log from completely compiling:

log from incremental compiling:

i have submit a issue: