The turnaround times are likely to be influenced more that they're loading 120T of fuel in a B787 vs 8T in a B737 (on typical routes). It's not taking them 2 hours to load the bags...
No it certainly is not taking two hours just to load the bags.
Turnaround includes loading and unloading of bags & passengers, food & drinks, rubbish, toilets checked/cleaned/replenished, mechanical checks to name a few. Q, and other airlines, leave the final food load quite late to ensure they're not carrying extra food that adds to fuel required as well as being thrown out at the other end when the carts get taken off. That's why if you buy a last minute business ticket, or swap to an earlier flight, you're warned that you may not get any 'catering'. Q have optimised their food carrying to match the typical proportion of meals consumed per number of business seats in use (for example). They don't just carry one business class meal per business seat on the plane - it is far more complicated. All this happens at a T - X minutes from departure.
With check-in closing the same time before departure regardless of the plane's size - that impacts the whole process.
And in relation to baggage, I would have thought B737 were a pain to load because they are not containerised.
No, B737s are much easier to turnaround requiring a fraction of the time of a B787. That's based on hard data not opinions, and Q's own presentations if you care to look into it further. That is why low-cost airlines predominantly operate single, not twin, aisle aircraft.
Hawaiian Airlines have got turnaround time for fully booked (128 passengers both ways with single narrow exit door) B717s down to as low as 13 minutes (I timed it while waiting to board). From doors opened to doors closed was a few seconds over 13 minutes. Perhaps the hot sun on the tarmac encouraged the handling team's speed.
All facets get taken into account including not just unloading/loading the baggage. With the B787 the balancing of the baggage load (for example) is a much more complicated process than with the B737.
Also the time taken to deplane/board passengers is longer etc etc. Loading the fuel is an independent process to loading baggage & passengers. Equally the time taken for cleaners/Q staff to check every seat is that much longer. Especially if the same number of staff are used for a B737 as a B787 to do the cabin cleaning/checks.