Mac – Manage iPhoto on external hard disk

Not sure if this will solve your problem but it might be a good starting point.

Try rebuilding the iPhoto library. You can do this by holding down the Command and Option keys while opening iPhoto. You will then need to choose the rebuild options. I think it would probably pay to select all of them. iPhoto will then work away for a long time and rebuild the library.

There is an Apple support web page that details this process further. I would normally say that you should keep a backup before making any changes like this but I guess you still have the copy of the library on the old iMac

If the rebuild doesn't help then I'm not really sure what you can do.

The weight of data (in gigs) doesn't really matter with iPhoto. That's just file storage. If you have a large HD with plenty of free space (30%+ and never less than 10%), it really doesn't matter.

iPhoto used to be rated for 250,000 items in the library, but in the most recent update it has the same format as Aperture, and that is effectively unlimited.

As to how well iPhoto scales: pretty well. I have a 40k plus library that opens in a couple of seconds. I've worked on Libraries in excess of 100k items and while longer to launch (one was almost a minute) it worked fine once open.

You can easily have multiple libraries:

Hold down Option while launching iPhoto, and from the resulting menu select Create Library. Bear in mind that only one can be opened/ searched/ accessed at a time. Again, unless you're getting very large indeed, I would avoid them.

Upgrading to Aperture is simple. Install it. Open the Library. Done.