Running on an I7 with 32GB of RAM and SSD and 4K screen, I find the interface delays the most irritating. Both stutter and waiting for effects of control movements to be displayed. Even just waiting for a histogram to display when selecting a photograph. I have Automatic Write XMP data selected because I use Bridge/ACR regularly. ACR is much quicker, probably because it is not doing so much coordinating in the background.
I don't have the slow downs you have with both LR4/ACR 6.7 (CS5) on a 2010 Mac Mini OS 10.6.8/8GB RAM, but I've noticed intermittent interface delays across all of my open apps (Firefox, CS5 Bridge, Photoshop) possibly caused by background activity when I traced the delays to Firefox automatically updating which requires I restart Firefox for them to take affect only I never get a dialog box alert that this is happening.
I just suspected that something was going on in the background after noticing the green LED "Ethernet" light on my AT&T U-verse box was blinking wildly but I wasn't downloading anything through Firefox. I checked "About Firefox" dialog box and sure enough it indicated it had updated without my knowledge and that I needed to restart for the changes to take effect. After doing so and spending a bit of time working in my open apps the slow downs stopped.
You might look into all open apps including any Adobe CC and others that do a check in on your system by just disconnecting your internet connection on your computer, restarting and see if the slow downs continue. You might have to spend a while working in LR with the internet disconnect to give LR and OS time to rearrange the furniture so to speak with API's that rely on this internet connection.
And just to be clear this is just speculation based on observation. Not sure if it would work for your system, but it's not too intrusive or a PITA to implement.