People will say this wholly about making more money and has nothing to do with delays though they couldn't be more wrong. Gate bags have been a huge problem under the current policy and it's now being addressed with this revised, and I think personally, fairer policy.
To clarify any misunderstandings, the current rule is that all passengers are permitted one small personal item AND one larger wheelie bag, up to 10kg, though only priority passengers are allowed their larger bag in the cabin - for all non-priority it's tagged at the boarding gate and put in the hold for free. The outcome of that, especially on low-booked priority flights, is you get a huge load of gate bags despite the overhead lockers being relatively empty, which causes the handling delays. The policy only really works well where there's a near 50/50 split of priority vs non-priority pax, but on average only 30% currently book priority (this should rise to the approx. 50% needed under the new rules).
The new policy means all passengers as a standard allowance can take one personal item (with the increased dimensions to 40x20x25cm +25%) that fits under the seat. Only priority passengers are permitted the larger wheelie bag, up to 10kg, on-board for the overhead locker (this will be capped to maximum 95 pax per flight). There will also be a new option to check-in a 10kg bag from £/€8 in the usual way at the bag drop desk - this largely replicates EasyJet's recent "hands free" motive. Any non-priority passenger who brings a second, larger bag to the gate will now have it put into the hold at the standard £/€25 baggage fee.
This policy should give far more control to strike the right balance of bags to essentially avoid the old yellow tagged "gate bags" altogether. People who want to take a second larger bag will know in advance if they can take it on-board (ie booking Priority) or to check it in the usual way at the bag drop desk (ie booking the new 10kg check-in option).