I agree with @MLBS
I would assume that tourist visas are processed much sooner because there's fewer things being checked than a PR visa application. Student visas have a bit more documents than tourist visas, but still less than a PR visa.
Visa processing time has always been case dependent, even before the pandemic happened. No one can clearly tell you why visa processing has become so slow, but we can speculate that maybe people working in the visa offices got sick (with COVID), visa offices closing down or reducing staff, redundancies, etc. Just because visa processing is conducted online and electronically doesn't mean that people working in visa offices are not affected by the pandemic. And it's not just Australia experiencing the slow visa processing, even Canada.
As far as I know, the Albanese government is making visa processing a priority, whether how soon they can organize (and train) new visa officers to speed things up is anyone's guess. Everyone will just have to wait and see, but at least we are aware that the government is doing something about it.