BFS-GLA - arrive at STD-40. 45 min block time. Bus to Glasgow City centre - 30 mins.
You would arrive 40 minutes before departure? With the state of security these days? Gate closes 25 mins before departure so you're giving yourself 15 minutes from the front door to the gate. That is ambitious.
Block time is until brakes-on. Add on time for airbridge and making secure until doors open. Then another 10 minutes through the airport to the bus station. Then wait for the bus.
There's easily another hour of hurry-up-and-wait you've elided from that air journey. I'm not saying ferries are fast ( not since they scrapped the HSS anyway ) but overall air travel is slower now than any time since the 1950s.