I may be mistaken in memory, but it seems that both the expansion of the EU (which mostly affects T1) and in particular the swiping of passports, which seems to more than halve the rate at which the IOs can process people, has been introduced without any commensurate increase in the number of staff provided at the EU desks at all.

10 years ago in T4 it used to be say 12 desks for non-UK (with a queue) and just one desk on the right-hand side for UK passports (who were the majority of pax). At the latter there was never even a queue, you just showed without stopping. Such is progress !