Do you believe the president deserves credit for the performance...?

He deserves partial credit. Optimism under Trump has been at historic highs and people have been seriously discounting how much that helps the economy. Deregulations, tax cuts, and increased spending have all contributed as well.

is he correct to claim a "turnaround of epic proportions"?

No. It's typical hyperbolic Trump. A turnaround of any proportions is a bit of a stretch, but the argument can be made. In 2015 and 2016, GDP growth had slowed and the stock market stayed nearly flat (until election night). Now things are trending back up. It's still too early to say anything definitive, but that won't stop Trump from saying it's the greatest thing ever. But Trump being the economy's hype man is good since it's self fulfilling.