Will applying Decimate Modifiers speed up Blender? I'm working on my first big paid project and I'm coming across a lot of problems.  I'm not sure what Blender can handle, but my project was something like 6 million plus faces.  I'd simply click a mesh and have to wait 3 minutes before it makes the selection.
Also, I'm trying to get where I can render this project in Eevee, and it's just been a complete nightmare.  I can only get it to render in Cycles, and only if I set it to render with CPU only.  There was a point where I could render in Eevee and to my surprise Cycles was faster, unless I disable particle effects for some reason.
So I'm trying to optimize my scene.  I've got Decimate Modifiers on almost everything, but I'm wondering if Blender is still calculating stuff before the Decimate Modifiers and if it will help things if I apply all these Modifiers.
So that's my question:  Will it speed anything up if I apply all these decimate modifiers?
If there would be a significant improvement, is there an easy way to apply all Decimate Modifiers on all the objects at once?
Any other advice to further optimize my scene would be a big help!
My Specs:

Windows 10
AMD Ryzen 5 3600
32 GB RAM
NVIDIA GeForce TRX 2060 SUPER
Will it speed anything up if I apply all these decimate modifiers?

yes (you can try by yourself btw).
I suspect you are not using correctly blender. If you have a lot of faces, that means that the mesh comes from a sculpt operation or photogrammetry, so you DON'T have to edit the mesh in edit mode. If you stay away from edit mode, you should be able to work on your file.