Airspeed v ground speed.
In still wind, put aircraft 3 miles (or minimum vortex spacing) apart on the ILS at 160kts airspeed and you’ll land 40-42 per hour.
In a 60kts headwind ground speed drops to 100kts so landing rate falls to 30-34 per hour.
As others have said, Heathrow applies Time Based Separation to help reduce the distance between vortex pairs but there are still absolute minimum distances applied, 2.5 miles between non vortex pairs for example, so TBS helps reduce the wind effect but it’s not a panacea.
Now think about the way Heathrow is scheduled at 42 per hour, every hour. When the landing rate drops to 32 per hour in strong winds the ‘extra’ 10 per hour have to be taken out of the system. Cancelling long haul is not an option so short haul gets disproportionately affected.
I’d rather my flight was cancelled the day before so I could make better arrangements rather than travel to the airport and cross my fingers it operates.
(The figures above are not intended to be absolutely accurate but estimated to prove a point)