Wifi performance on Mac mini

The wifi performance on my Mac mini is driving me crazy. For reasons I can't figure out, it will go through periods where the wifi speeds drop to almost nothing and no internet pages will load. It is still connected to the router, and the signal strength shows full bars. Our iPhones never have this problems with the same wifi. Even when the internet is crawling on the Mac, the iPhones will work without problem. After a while, the speeds will get back to normal.

I have tried everything, but can't find any problems. I will close programs, close extra web pages, etc. Nothing I can figure out that would cause the speeds to go from normal to nothing. The normal speeds are about 25-40 mbps, nothing great, but it's better than 0.

Thanks for the help!