> For me it all made sense when I heard that IQ/g-factor basically vanishes in the absence of time pressure (heard if from Richard Haier on Lex).

That cannot be true as there are valid IQ tests that doesn't have a time component, and people don't all score the same on those. He must have meant something different than you think.

For example Raven's matrices was originally an untimed test, how can that be if there is no G-factor in untimed tests?