Stock Messaging App Locks and is Slow
If you are coming from an iPhone and looking for a better messaging app, check out Go SMS Pro and use an iPhone theme.

This app allows you to customize several things while still looking very similar to the iPhone's messaging interface. I always install Go SMS over the default messaging app and you might just want to give it a try, especially since you are coming from iOS. There are other plugins that are all free that allow you to view iOS emojis and other cools things like that.

Let me know if you still have the same issues with this app as well.

btw, congrats on smartly making the switch ;)