Exactly. It was MOS that lagged, not "transistors" or "computers", really. Transistorized microwave circuits (analog, not digital) in the GHz range were operating as early as the 1960's too.

MOS is just hard. It's hard to fabricate, it's hard to design (i.e. you need computers to make computers!), it's hard to scale. It required new chemistries be invented and new circuit design techniques to manage the (really tiny!) components. So for a long time you could have a big circuit in discrete bipolar components which ran fast or a tiny one in MOS which was slow.

But MOS was tiny and MOS was cheap. So by the early 90's it had caught up in speed with all but the fastest discrete technologies (GaAs was "right around the corner" for like two decades as it kept getting lapped by routine process shrinks -- now no one remembers it) and the market abandoned everything else.