My iPhoto Library is about 900GB. Its even causing my $4,800 macbook pro to lag

It will definitely take a lot of processing before Photos will be able to browse your library quickly. Keith already mentioned the indexing, but Photos for Mac is using artificial intelligence algorithms to analyse your library, so it can search the photos and videos by what is shown in them. A new and powerful Mac helps a lot to get this analysis done quickly, but with 900GB of photos and videos it may still take more than a week or even longer to prepare your library. The processing time for the analysis of the photos and videos is roughly proportional to the number of media files and the size of the media, particularly for the videos - the whole video has to be scanned for faces and objects.

Give your Mac the opportunity to get this done, while you are not using your Mac; keep it running over night.

The last leg -from 70% to 100% will take the longest time.

But look at the "All Photos" tab at the status bar there. Is there any indication about photos that cannot be processed? Sometimes a single video with an unsupported codec , audio files, PDFs, or a file with missing originals can cause Photos to be stuck on this item trying over and over again.

Check the status of your library in Photos for macOS - Apple Support

While Photos is preparing your library you may want to check, if you are having problematic files in your library and remove them.

First let's be clear, you have a Photos Library, not an iPhoto Library.

If you just got it, sometimes it takes some time for all the indexing to complete before it gets snappy again.

You could try to rebuild/repair your library.

You will see that Photos is scanning your library, if you are looking at the Library > Days tab in Photos. Below the Days you will be seeing an endless loop, showing "Analysing Library, Curating best photos, detecting duplicates, composing layout". Leave photos open on this view, so you see, if something is happening.

It will quickly show a progress and then probably and for a long time on 90% or so.

Turn off the sleep mode and just let the screen saver kick in. I use a blank, black image for that. and leave Photos on but in the background, i.e. click on the Desktop so the Finder is the active app.

Don't put it in the bag at night while you let it do it's indexing thing.

Yes, you need to turn off sleep mode, and have it connected to power. It wont harm to leave the photos app open also.

There should be a checkbox:

"Prevent computer from sleeping automatically when the display is off"

I think this one stops sleep.

Having the mac plugged doesn't harm the battery. In fact it is better for it. (Battery life is "consumed" by charge/discharge cycles). When the mac is plugged in, once the battery is fully charged, they battery management will stop charging and run from mains power.

goingape wrote:

So far after 6 hours plugged into AC power the status bar is at about 10-15% complete. Should I worry about damaging the battery keeping it plugged in for so long? Also I dimmed the screen almost all the way and picked a darker screen saver. There wasn’t a black one to choose from.

macOS Monterey supports an automatic Battery health management. Here is some background information on the Battery Health of MacBooks with Apple Silicon: About battery health management in Mac notebooks - Apple Support