iMac, Macbook Pro suddenly running slowly

For the last few weeks all of my iMac (3) and MacBook Pro are running slowly. All different ages and operating systems. One is less than a year old. The spinning wheel has become a constant disruption on all of them. Some pages never load or only partial load the pages. I have done some investigating, I am no tech however. I am at a loss. I question my wifi since all exhibit this behavior.
One of my iMac's has the blue screen. I booted to safe mode but nothing tells me what to do next to save the unit or correct the problems. Can someone assist?

Can you give us the specs on each of the machines? It sounds like there may be a common cause but it's also possible that the issues/causes may be different for each computer.

Are these slowdowns only occurring at certain times (such as when using the Internet)?

Welcome to the forum. It would help if we knew what the machines are, what version OS X, etc. And since you mentioned pages loading slowly, one thing all three have in common is your network connection (modem, ISP). Have you checked with your provider to make sure everything on their end is ok?