A year or two ago, I installed a new, single handle Delta kitchen faucet with a "Diamond Seal Technology" single handle cartridge. In the last couple of months, when the handle was pushed over to select hot water, the faucet would flow normally for about 20 seconds and then reduce the flow to just a minor amount. This slow flow happened just when the water was starting to feel warm,.
I finally got ready to replace the cartridge. When I removed the handle (with a 6mm Allen wrench on the set screw behind the red and blue plastic plug), I got ready to take out the valve cartridge when a delay came up and I had to reassemble everything. I turned the water back on and checked the faucet. To my amazement, the hot water slowing effect was no longer present.
The only thing I could think of that might have remedied the situation was that I might have turned the shutoff valves on a bit farther than they were before.
At any rate, if you have this sudden flow reduction problem with a Delta faucet, try shutting the valves off under the sink and then opening them up all the way.
A note of caution: If you haven't turned off the shutoff valves in a long time, they might (1) be difficult to shut off all the way because of mineral buildup or (2) start leaking around the stem because of deterioration of the stem or the washer around it. When I replaced the kitchen faucet a couple of years ago, I had to replace both shutoff valves, too, because they were both corroded and leaking. That made working on this repair much easier.
If you do need a Delta Diamond Seal Technology Single Handle Cartridge, the part number is RP50587. If the flow down issue returns, I have a spare ready.