Who decides on those 20 minutes anyway?
This is good question. On Bitcoin Wiki, 20 minutes only mentioned since early 2013[1]. But afterwards i found out Gavin Andresen propose this[2] and ask pull request some time later[3].
I checked the last 2 days, and to my surprise only 115 blocks per day get mined.
Mining difficulty on testnet keep rising which discourage some people to perform mining, where people with CPU/GPU only expect to earn tBTC from 1 difficulty after 20 minutes.
I'm surprised because the total block height is much higher than for Bitcoin.
Probably because now we have separate testing network dedicated for newer Bitcoin feature (e.g. signet and segnet) and in past people attempt to attack/stress Bitcoin testnet by mining with tons of ASIC.
[1]
https://en.bitcoin.it/w/index.php?title=Testnet&oldid=35502[2]
https://bitcointalk.org/index.php?topic=50223.msg627957#msg627957[3]
https://github.com/bitcoin/bitcoin/pull/686