Is it possible to decimate multiple objects (layers?) at once?

Hi! I’m new to blender and this forum, I hope this is the right place to post this question:

I have a laptop with far too many segments, somewhere in the neighbourhood of 500,000. There appear to be 600 layers in total, which make up a variety of materials/objects in the model. Rather than go through and decimate one layer at a time, is there anyway to do them all at once? It would be beyond tedious to go through each layer in this model, and individually decimate it.

Help!

Thank you!

You would have to join them into one object. Select those objects you want to join and use Ctrl+J