Am very puzzled by this. LHR must be operating at what, 25% of capacity at the moment? Why are things being timed to the second then in terms of approach spacing that seems to necessitate exiting the runway as early as possible after landing?
As LHR is down to one runway for a few months now the current landing rate can't be as high as it was before when using segregated takeoff and landing runways since space also needs to be left for take offs anyway. Tbh it is not my experience that approaches are spaced as tight as pre covid as a matter of course. The lower usage is shown by the fact very little holding tends to be used now just because arrival levels are not high enough to need to do it. I don't think it is a conscious decision not to use holding, rather that it isn't needed as a tool currently. Pre covid landing rate was around 40-42 per hour (so averaging around 90 seconds between landings) and I can't imagine it is getting higher than 15 or so at the moment. In fact it is very odd watching and seeing nothing arriving for 5-10 minute periods sometimes when compared to how busy it was before.
A9W would usually be the expected exit point for most short haul aircraft landing on 09L (similarly when landing on 27R it would be A9E). Braking might be heavier depending on weight of the aircraft, higher ground speed when landing, touching down more towards the end of the touchdown zone etc, and not necessarily because the flight crew have been told to expedite exit. I have been in to LHR around 20 times since the end of March and I haven't noticed any trend towards harder braking or exiting at exits before those normally used on each runway. (EDIT: perhaps the A320/321neos new braking system ends up giving a harsher passenger experience?)
I think the OP mentioned their departure yesterday lining up whilst the preceding aircraft was still rolling? That would be fairly standard, especially pre covid since there was rarely an aircraft on the approach to that runway. Down to one runway it must be a little less common I guess, but it is still happing as there isn't a constant intensive stream of arrivals. For a truly busy one runway operation you only had to look at LGW, it was very impressive seeing departures and arrivals being weaved together so tightly whilst sat in the lounge - often with a departing aircraft lifting off just as an arriving one was to touch down some seconds later.