Do I understand it well, does LR always or sometimes load during Developing (editing) from the disk where the RAW image is stored in this case an external Disk.Or does it load most of the time from the ACR cache, then there should not be a delay.
Do you mean with ACR cache the Camera Raw Cache which you can adjust in Preferences and Camera Raw Cache Settings? I adjusted it to 200GB.
I think a little clarity is needed here. It's not a case of Develop loading EITHER the ACR cache entry OR the original raw file....in most circumstances it will do both!
The ACR cache entry for a raw file is created whenever you have Lightroom render a standard or 1:1 library preview, it is stored in the ACR cache which by default will be on the system drive (but can be user-changed). The size of the cache can also be user set, and it works on the basis of the oldest will be removed if the cache is full in order to store the latest entries. The cache entries will typically be in the 500kb to 1mb range, so a 20gb cache will hold around 20-40,000 entries.
So unless you only create minimal previews on import, the ACR cache entry will have been created before you try to open it in Develop. In that situation the Develop file opening process is as follows:
1. Read the Library preview, which is the first thing you will see, but at this stage sliders are not yet activated.
2. Read Smart Preview. If you haven't rendered Smart Previews, LR will open either the ACR Cache entry OR (for DNG files only) the Fast Load Data if it has been rendered. The ACR Cache entry and the DNG FLD are basically the same thing, but as the latter is stored in the DNG file, reading that could be slower if for example the original files are on a network or slow speed external drive. For files which have already been edited the preview from the SP/ACR?FLD entry will first be updated into the system cache with any existing develop edit instructions. This preview will be the second thing you see.
3. Whilst the above is going on, LR will also be reading and converting the original raw file (+ applying any existing develop edits) to create the "scene referred" full-size Jpeg preview which is the last thing you will see in this opening process, and which is stored in the system cache during the edit process.
You'll probably not notice much difference between the first two steps, but at the end of step 2 the develop sliders will be activated.
It is possible, if you look closely, to determine a change when the final stage preview is loaded and displayed on screen.
My take on the above is that having the smart previews (if you've created them) or the ACR cache on fast drives is good. Having the original files on fast drives is far less important (in Develop terms), as the transition to the final stage Develop preview will be almost seamless so it probably won't matter if it takes a few seconds longer. But it might matter if the original files are now DNG and Fast Load Data is being created.
Of course all of the above pre-dated the new performance improvement in LR6.6, which could mean that loading the next image is virtually instant.
Hmmmm....maybe on reflection I didn't bring that much clarity, but it's important to understand what goes on "under the hood" if you're considering file placement options/strategies.