>>Is there any easy way of slowing down everything in the scene?

Well, yes, and no. It depends on what you wanna do.

You've stated that you want EVERYTHING to slow.

You can do this by rendering out your sequence at a higher frame rate like say, 90-120 fps, and then in your compositing app, blend the "normal" speed footage with the "slower" footage so that way you can "ramp-up" into the slo-mo sequence without it being to jerky.

However, if you actually want to keep the same frame rate, and make everything slower, I believe you can add a timewarp to things in the Hypergraph to do something like this.

Check the docs for "Time Warp". Also, search SM and Highend for the same text, and you should get a LARGE amount of reading on this very topic that will lead you in many directions

Good luck!