Thanks for the thoughts - there's no commute per se, as it's online (although realistically I wouldn't feel comfortable doing an interview for a different firm from the firm's office, so I'd need to find a location elsewhere, which would probably add around 15 minutes to my downtime).
As I understand it the 90 minutes is comprised of a 45-minute interview and an assessment: it's not clear whether the assessment can be done separately; I've emailed to ask, but I suspect not - a 45-minute interview seems more doable, but 90 minutes seems excessive.