Here's the clue that catches my eye:
"This database (like all of the others) was converted from FMP 6..."
Open Manage | External data sources for this file and examine the external file source references. Brace yourself, files from that era and older often made a very messy "hash" of their external file references. Look for two possible sources of trouble in these references.
Asterisks. Using asterisks to link to a hosted file is still a valid syntax, but the files so linked open very, very slowly.
Invalid IP addresses. If a file attempts to access an external data source on a hosted file via an IP address that does not exist or at which the file is no longer located, it can take FileMaker a very long time to determine that and move on to the next file reference in the data source.
That was it! Thank you so-o-o much. The external data sources were pointing to a Mac hard drive (bizarre) and other files that are no longer even in use. Once I deleted them it opened perfectly. Thanks again!