Parallelise your Grails functional tests! Put this in your scripts dir.

TestShard.groovy

import org.codehaus.groovy.grails.test.support.GrailsTestTypeSupport
import org.codehaus.groovy.grails.test.GrailsTestTypeResult
import org.codehaus.groovy.grails.test.event.GrailsTestEventPublisher
import org.codehaus.groovy.grails.test.GrailsTestTargetPattern

includeTargets << grailsScript("_GrailsClean")
includeTargets << grailsScript("_GrailsTest")

// grails t-s shard=1 shards=4

target('default': "Run functional test shard") {
  depends(checkVersion, configureProxy, parseArguments, cleanTestReports)

	List<String> tests = new SpecFinder(binding).testClassNames.sort()

	int shards = (args =~ /shards=([0-9]*)/)[0][1] as Integer
	int shard = ((args =~ /shard=([0-9]*)/)[0][1] as Integer) - 1

	int shardSize = tests.size() / shards+1
	List<List<String>> shardedTests = tests.collate(shardSize, true)

	testNames = shardedTests[shard]
	phasesToRun = ['functional']
	testOptions.https = true
	testOptions.war = false

	allTests()

}

class SpecFinder extends GrailsTestTypeSupport {

	SpecFinder(binding) {
		super('name', 'functional')
		buildBinding = binding
	}

	int doPrepare() {
		0
	}

	GrailsTestTypeResult doRun(GrailsTestEventPublisher eventPublisher) {
		null
	}

	List<String> getTestClassNames() {
		findSourceFiles(new GrailsTestTargetPattern('**.*Spec')).sort{ -it.length() }.collect{ sourceFileToClassName(it) }
	}
}