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.