Overall, it isn't, but there are certain issues at certain times. In particular, check-in / bag drop desks on weekend mornings and part of the afternoons, and immigration when the eGates are closed because Border Force is doing manual checks on the Passenger Locator Forms (generally earlier in the day).

Both could of course be solved (or at least improved) with more staff. BA could obviously choose to bring more staff for check-in back from furlough if it desired (no shortage of desks available), I don't know what the situation is with Border Force staffing.

Apart from that, my experiences of T5 in the last few weeks, both arriving and departing, have been that there is plenty of capacity. Maybe there are other pinch points at certain times (security?), but not that I've seen.