Always ON Performance

Always ON performance impact if secondary server is of low configuration compared to primary server.
Synchronous mode always on configured on sql server standard edition.

I won't claim to be a definitive source, on this topic, by any stretch of the imagination... So follow up and verify...

If you're running AO asynchronously, there should be NO impact... Just be aware that asynchronous does leave a window of potential data loss in the event you loose the primary.

In synchronous mode... As long as the secondary is able to consume the log file info from the primary at the same rate the primary is producing it, then it shouldn't be and issue. From that aspect, the quality and throughput of the network connection will be very important.

But... before you dial back on the secondary... Ask yourself WHY you're setting up AOAG in the first place?... If it's not able to handle a full work load in the event of a failover, why bother with it at all?
That's not to say the secondary needs to be fully spec'd out like the intended primary... but I'd want to have absolute confidence that my it cold survive in the primary role for as long as I needed to get a new primary stood backup, tested, and put into service... Just my 2 cents.

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Probably a common decision where "unlimited budget" is not available?

"Lets get a shiny new Primary server, and that old one can become the Secondary"