Is the problem primarily during the October November & April May shoulder seasons, or is this a dead-of winter phenomenon?
If the former, the response is slow because it's a high-mass system, with a huge volume of water, which takes time to heat up. Hopefully the boiler is plumbed with boiler-bypassing or similar to protect the boiler from destructive condensation during those long cold-starts. During sustained operation you don't want the water returning to the boiler from radiation to be colder than 130F or it'll be damaging the heat exchangers in the boiler. If it keeps up during the coldest nights in January the boiler is not undersized- it's probably "right sized" or slightly oversized for the peak load, depending on your insulation levels, glazing, and air-infiltration levels, but can't deliver a rapid-response on cold-starts due to the high thermal mass of the system. I have similar age & sized house in central MA, with double-hungs + storms that I've been tightening up and insulating. Without going hog-wild my design-day heat load has been reduced to ~ 30k, but it was probably 60-70K when I moved in. Your design temps are proably no more than 15% colder than mine, so a 75-80K true coldest hours heat load might be about right, or might be higher than reality if the place has seen insulation & window upgrades. Plumbed properly, high thermal mass in the system would be your friend for both comfort & efficiency.
Depending on how the whole thing is laid out, you may need to plumb in mono-flow tees, partially bypassing the first-floor radiators to have more/warmer flow to the higher floors. Or there could be sludge/rust crud creating flow issues to the upper floors (but unlikely with big-bore pipe like that). If it's a manifold/loop configuraion, ball valves on the loops can be used to adjust balance. But without a system schematic it's really hard to say what the problem is, and how to fix it.
Did it EVER run hotter?