I think you're starting with the wrong assumptions.
What objective metrics makes you think there is any slow down in innovation?
You may feel like progress is slow, but it's very human to have irrational feelings :)
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.