I don't know if this is more efficient, but to my eyes, it's a bit simpler (YMMV)
def data = "session=234567893egshdjchasd&userId=12345673456&timeOut=1800000"
def map = [:]
data.split("&").each {param ->
def nameAndValue = param.split("=")
map[nameAndValue[0]] = nameAndValue[1]
}