Thank you KevinTheJedi . The tables were MyISAM; unfortunately, converting them to InnoDB does not seem to have helped. It's on a capable dedicated (8-core 16GB ram) server that's not maxing out... but on the other hand the MySql (5.6.49) version is probably right at or around the minimum MariaDB equivalent.
I have integrated the information from closed tickets into our in-house CRM app, which is very helpful to staff to be able to look back at a customer and their history of help... So, we would really like to keep old data. And while 15,000 closed tickets is a lot, there must be many folk who have many more?
It's funny that osTicket v1.10 runs very fast, but v1.15 is so slow, bummer. We will be upgrading our MySql (and server) after the holidays, so hopefully that'll help... Otherwise, at the moment, I guess we'd be stuck staying with v1.10.4...
Any further thoughts welcome. Thanks again!