I use both LR and PS extensively. Probably 80% of my images have a round trip to PS.
PS is great or required for combining multiple images, precise control for masking and selections, retouching, filters, geometric transformations (e.g. liquify, puppet warp, split warp), guides & measurement tools, CMYK, sharpening, blend modes, custom brushes, plugins, etc. I also like the way some adjustments work better in PS (selective color, color balance) compared to the tools in LR. In some situations, I find the layer stack, and ability to group layers, extremely helpful especially in complex edits.
Technically, any processing you do in LR can be done in PS because of the common camera raw engine. But LR is great for syncing edits, many times I'm not sure exactly which image I want to use without doing at least some processing. LR / Camera Raw is also great for algorithms that work best on the raw data (denoising, capture sharpening, highlight recovery). I try to do as much as possible in LR.
LR sometimes gets super slow with large panos. I haven't noticed the same slow down in PS. However PS is really slow to save files, especially when they balloon up with the layers and you need PSB compression turned on.