I've roughly halved the number of database queries used, I think this will solve it.

Same issue on RC33 and I wouldn't consider my disk slow (though the installer does skip file checking because it thinks it is). It's like there is no attempt to initatiate the data transfer by the browser (Firefox) on Step 6. Takes a few tries for it to do so.

:(

Is it an SSD or a traditional magnetic HDD? I've observed many times Windows is much slower accessing small files that Linux, I'm not sure if that's a disk caching thing or something to do with how NTFS works.

How long until it white screens in seconds?

It's a standard HDD but ocportal and early RC's didn't consider the disk slow. It pretty much white screens as soon as the Step 5 autoproceed counter ends. I have to hit back and let it try again until it takes. Once it does it takes a few seconds to complete the actual step.

I just tested on WAMP running on a VM of Windows 10 and it went through okay.

Okay, perhaps it's a Bitnami Wampstack thing or my machine, though this a new PC with twice the amount of everything my old one had so strange to be encountering the same issues I had on the old machine. If nobody else is having this issue, I guess I can live with it as it eventually works.

Now fixed.

Bitnami set a Zend OpCache extension setting unusually high
opcache.revalidate_freq=60

This broke the config file loading within the installer. It was initially blank, Composr would load it for default settings, Zend OpCache cached it as blank, Composr wrote the file, but it would take 60 seconds before it would decache so the next screen (when it was needed) failed due to the stale cached blank version.

Ahh right, glad you tracked it down and fixed :)