Just setting up my hub but cant seem to find the simple lighting app? Does it need to be added manually?
It is now called Simple automation rules.
It also does need to be added. Go to the top right hand corner and click Add Built-In App. You can search or browse that list to get familiar with all the available Built-In Apps. Don't install them unless you want try it out, and remove any you are not using.
Out of curiosity do you know if it supports dimmers, I'd found the turn on/off but couldn't get the dimmer to work so thought there might be a lighting specific app that had that.
Using a Aurora Dimmer with the generic driver
Does having ones your not using cause slowdown? or is it just good house keeping?
I cannot speak officially on slowdown, so house keeping. I've never been a fan of bloatware (stuff hanging around unused taking up space)
You can set dimmer level with either RM or Simple. Faster, basic stuff with SA more complex rule in RM.
They should update the documentation here then: Hubitat® Simple Lighting - Hubitat Documentation