Why is Lightroom so much slower than ACR?

Question

This is a personal curiosity question. I'm not trying to solve a problem (mainly because I don't think there is a solution).

The develop module of Lightroom, from what I understand, uses the same engine as Adobe Camera Raw. Why, then, is the develop module so much slower than ACR? Specifically, I can adjust sliders in LR, and there is almost a half-second lag between the movement of the cursor and the slider moving. (This is with an i7 processor and 16GB ram). I can process the same photo in ACR with no such lag.

Answer 1

I discovered it was the graphics driver. It needed updating and there is still a little lag, but it is almost unnoticeable.

Answer 2

Lightroom has a cataloging and non-destructive edit stack constantly running in the background. It accesses files, writes their states to it's catalog and so on. Opening Task Manager in Windows while editing a photo shows disk usage spikes by Lightroom (I cannot show it here, but it is noticeable in the expanded Processes view of WTM). The slower the disk, the slower LR is to perform. E.g. I have a 5400 rpm HDD (It's 6 years old almost, don't judge). My friend has a 7200 rpm drive on his laptop. His LR performance is noticeably snappy, whereas ACR in Photoshop works about the same. Someone with an SSD will have an almost buttery smooth LR experience, provided it's set-up correctly.

Note: ACR also writes states to the location of RAW files as .xmp files, but these are for individual files, there is no collation of files into collections and such. LR instead has one single catalog file as well as individual stack-files (I don't know the exact technical term but this is close enough) for each photo. This tremendously increases disk usage. And disk is always slower than RAM. Hence ACR is faster than LR, but lacks the organisational features.