Yes, That's one of the many software problems which riddle this device. I am using Nova Launcher and I've had the same problem since i have purchased this phone.
-Instruction for other users who have the "slow screen wake" issue and are searching for a temporary fix, as already suggested by OP-
This temporary fix will work until you'll turn off/restart the device. In order to have a regular wake up for the screen with a 3rd party launcher, you need to go to settings-apps-preset apps, then select "starting app" (or something like that, I'm using the Italian version of the Os), select "Asus launcher" and after that select again your 3rd party launcher. Problem fixed, now the screen will always wake up at regular speed until you'll power off/restart the device. Repeat again after rebooting your device to have your temporary fix again.
Let's hope developers will see this thread and will permanently fix this problem together with all the other problems which this smartphone has since day 1.