Probably corps. I was working at Factset in the early 2000's when there was a big push for it and I imagine the same thing was reflected across every Microsoft shop across corporate America at the time, which (at the time) Microsoft was winning big marketshare in. (I bet there are still a ton of internal web apps that only work with IE... sigh)

Obviously, that means there's a lot of legacy processes likely still using it.

The easiest way to improve the situation seems to be to upgrade to a newer version of XSLT.

I recently had the occasion to work with a client that was heavily invested in XML processing for a set of integrations. They’re migrating / modernizing but they’re so heavily invested in XSL that they don’t want to migrate away from it. So I conducted some perf tests and, the performance I found for xslt in .NET (“core”) was slightly to significantly better than the performance of Java (current) and Saxon. But they were both fast.

In the early days the xsl was all interpreted. And was slow. From ~2004 or so, all the xslt engines came to be jit compiled. XSL benchmarks used to be a thing, but rapidly declined in value from then onward because the perf differences just stopped mattering.

In the browsers.