While progress is difficult to quantify (and is therefore often subjective), I certainly don't think the argument about whether or not there is a slowdown is "irrational"

Here is a list of scientific discoveries / innovations made during the 1800-1950 period off the top of my head:

- Steam engine - Indoor plumbing - Penicillin - Electricity - Internal combustion engine - Transistor - Theory of evolution - Heavier-than-air flight - Electromagnetism - Splitting the atom - Skyscrapers (The Strasbourg cathedral was the previous record holder for world's tallest building. That cathedral was started in the late middle ages) - Metro systems - Vaccinations - Jet engine - Typewriter - Dishwasher - Telegraph - Open heart surgery - Plastics - And so many more...

Since 1950, the consequential innovations I can think of are - Artificial satellites - Computers - The internet - Smart phones

Certainly all four are meaningful but are they, combined, as consequential as what happened over the previous decades?

I think that one measure you could use is whether life would be more unrecognizable between 2020 and 1950 or between 1950 and 1880. I would argue that 1880, when life expectancy was still under 50 years at birth, when manure pollution was a major problem in NYC, and right before that city was electrified, was far more different for the average person than 1950 when we lacked our modern electronic niceties.