Case for market efficiency and against

I agree on academia on the case of market efficiency because its almost impossible to detect any price inefficiency. The concerted effort of all investors to find these inefficiencies is so tremendous. I also posit that there are some inefficiencies in the markets. But they are so few and occur so infrequently, one has to be very patient to find them.

Taking 'price efficiency' to mean that prices immediately and fully reflect all currently available information, it follows that you are not going to detect any price inefficiency unless you have inside, i.e. not widely available, price sensitive information. Then it is easy to make money at the expense of less informed others and this is why in most financial markets insider trading is a criminal offence.

As to there being some inefficiences in the market, I would agree there are, but it's generally not because of the availabilty of information, but because of the less-than-perfect reaction of investors to price sensitive information. Not all market participants react immediately to news, and there is no guarantee than the initial analysis (and therefore reaction to the news) is 100% correct. Equally, how news is evaluated varies across market participants according to their differing beliefs. Eventually an equilibrium consensus (and so new equilibrium price) will occur but full efficient price reaction to news takes time, and inefficiences exist in this period of adjustment.

This explains why prices can jump violently back and forth on news releases (imperfect and/or differing initial analysis), and why prices can trend for extended periods (time needed for a final equlibrium price to be arrived at as participants react at differing rates).

So to sum your post, the market is tending towards equilibrium in a disequilibrium manner. I like that.

My perception is that the fx market tends to overreact heavily to most news releases. Thence can I not posit it as ‘efficient’ — not in the short time-frame at least.
It seems to have inefficiency built right in.

Market inefficiency is more easily detectable on shorter time frames.

This is where hft quants excel and successful scalpers make their living.

I don't think there is any difference between timeframes. I believe the markets are fractal. The same things happen on different timeframes and also across all markets.

For instance, I believe USD was overvalued for practically the entire 2015 and 2016, and its slump in the spring and summer of 2017 was major correction to that. So I might see it as a case of inefficiency taking years to correct. If the markets are fractal, it would imply there are inefficiencies in them spanning decades, too… (They’re just difficult to see, and close to irrelevant for trades or investment in shorter time-frames.)

Distribution of returns is really different on low and high timeframes, I guess it's because of noise which dominates the moves on low timeframes like 1M or 5M. But it needs additional check I guess.

the key is "being efficient at what"? one is efficient only if it is very good at what it has to do.

imho, the goal of the market is not to set a price, hence considering the academic definition the efficient market as being able to set the "best price reflecting all and every condition at time t" is a wrong and pretty useless view of the market (hey... that's academia, that's theory ).
In practice the goal of the market is to facilitate trade (setting the price is a consequence, not the goal); in short it means making possible as many transactions as possible and in that sense, I think the market to be a damn good very efficient device (though there is still room for progress).

There are inefficiencies spanning decades too. Like long term cycles etc. 2008 was one of them, it was the burst of a bubble that had been developing for almost 4 decades.

It's because of the greater impact of commissions on results .

It's quite fascinating really,the markets efficiency is part of the reason for it's inefficient behaviour, If it was consistently efficient a lot more people would make vast amounts of money ,therefore it would no longer be efficient.