Tom Newton wrote:

I live in NY where it's cold...

How cold is it?

The kitchen is 12'x17' and, unlike the rest of the house, sits on a slab
extension out back...

...yesterday, they finally hooked up the radiant heat to the boiler

How many Btu/hour?

After running it for two hours at 180 degrees the damn floor is still cold...
and the return water is barely luke warm!

How many gpm? What's the floor temp and the return water temp?

...although he was able to jack hammer away the very poor concrete laid by
the previous owner, there was some incredibly hard concrete below that that
made it impossible to go any more than 2 inches deep (the house probably had
a patio or landing out back originally, the house is 90 years old)

....2" of new concrete over 6" of old concrete over soil or stone might be
8"/12"x12'x17' = 136 ft^3 with 25x136 = 3400 Btu/F of thermal capacitance.
With minimal heat loss to the ground underneath, a 50K Btu/h boiler might
warm it 100K/3400 = 29 F in two hours.

At this point, 4' of R10 perimeter insulation seems like a good idea,
along with baseboard radiators...

Nick