Time to generate blocks isn't the same as what happens to price. They are loosely correlated, but not the same thing. Price is as much psychological as technical.

However to answer the question: If mining in China went offline, it would affect time to blocks. However, difficulty adjusts every 2016 blocks (about 2 weeks), so if China has 75% of mining, at worst, it'd be 8 weeks before everything went back to normal in terms of block times. Bitcoin Cash actually has mechanism to adjust far more frequently, within a few hours if necessary. (but also subject to miner manipulation)

Bitcoin adjusts block generation to 10 minutes irrespective of the miners. What does change is difficulty, hashrate and transaction fees.

The bitcoin hashrate has remained more or less the same in last 2 months: https://blockchain.info/charts/hash-rate?timespan=60days

This is in contrast with last 3 months where the hashrate nearly doubled: https://blockchain.info/charts/hash-rate?timespan=180days

Fees peaked around 25th August but still pretty high: https://bitinfocharts.com/comparison/bitcoin-transactionfees...

Hashrate has not remained constant:

- 7 day avg of last 3 mo: https://blockchain.info/charts/hash-rate?timespan=180days&da...

- 7 day avg of last 2 mo: https://blockchain.info/charts/hash-rate?timespan=30days&day...

Going from 6 to 7.5 EH/s it's not more or less the same (it's a 25% increase)

The exchanges don't typically create a blockchain transaction when you trade, so block mining times do not factor in here. It's just an open market with buy/sell orders that anyone can place at any time.

I'm not sure which of us has misunderstood something, but I don't see why the speed of mining would affect the reaction time of the market.

This doesn't happen in real time, but adjusts every 2016 blocks. So loss of hash rate will temporarily affect block generation time until the next adjustment.