It's 20 minutes already into the show
I would move the word already so
It's already 20 minutes into the show
It's 20 minutes into the show already
20 minutes into the show is really a single unit of speech. Imagine replacing that with Tuesday afternoon. you wouldn't consider saying
it's Tuesday already afternoon
you would say
it's Tuesday afternoon already!
it's already Tuesday afternoon!
In the comments I was asked if this was a natural wording, and I would say yes it is I can fully imagine somebody saying it. I presume the person is speaking American English as in the UK things on the telly are called "programs" not "shows".
But honestly if I was talking to somebody about it I would say thing like:
We are already 20 mins in
We have missed 20 mins already
But I have been told I am a terrible communicator in everyday life
to expand upon the statement above "We are already 20 mins in" to expand this utterance into a sentence I would write formally:
"Oh No the show started 20 minutes ago, we have missed so much of it.".
"We are" means that we have now been watching this for 20 mins.