Guys who read text messages but reply 10-20 min later?

So on the iPhone you can see when they read your iPhone messages

And sometimes a guy will read my message but won't reply for 10-20 min after

I know you will say he's busy, but I honestly think it's because he's trying to come up with a reply

True or not?
Guys who read text messages but reply 10-20 min later?
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