History of blockchains. Appearance of ASICs.
To see how close the Bitcoin blockchain is to the goal of creating a block of the blockchain you can visit the official blockchain explorer at https://blockchain.info/ and check the “age” column on the top of the page. Age means the time between the moment you arrive on the page and the time of creation of a block. For example, if you see the following in the age column: 2 minutes, 22 minutes, 26 minutes, it means that the Bitcoin blockchain has created its latest block two minutes ago and is currently working on a new block (which it always does). Before that, it created a block 22 minutes ago, which means that it took the network 20 minutes to create the latest block (22 – 2). Before that, it created a block in 4 minutes (26 minutes ago minus 22 minutes ago means that there was a block it created in just 4 minutes). The principle and approach work in the same way on other blockchains, too. You can see how often a blockchain creates blocks by visiting a blockchain explorer for a particular network. Here, for example, is the explorer for the Ethereum network https://etherscan.io/ and here is the explorer for the Sia network https://explorer.siahub.info/ Some blockchain networks, such as Ethereum, create blocks much faster than the Bitcoin network does and some include much more information on their blockchain explorer pages than Bitcoin, so the look of a blockchain explorer may be a bit different, but the way the technology works would be very similar.
Because the blockchains do not create all blocks with exact same speed, the exact times when the networks adjust the parameter of difficulty may vary, but there would be other parameters that indicate when the adjustment will happen.
As explained above, on the Bitcoin network this happens every 2016 blocks. If it took the network more than two weeks to create the blocks, the difficulty goes down (to try and take the time of block creation down and bring it to exactly two weeks). If it took the network less than two weeks to create 2016 blocks, it means that the job is too easy or there are too many miners and the difficulty goes up. When Satoshi Nakamoto created the Bitcoin network in 2009, hardly anybody knew about the network. The price of Bitcoin was less than a penny and there were very few miners on the network. Because of this, simply using a home computer was sufficient to be a miner and to be getting 50 bitcoins for mining a block of the blockchain. There have been people who have tried mining and then gave up, which is why today you will hear about stories like this the one about the man who threw out a computer with over 1,000 bitcoins now worth $80 million that he mined in the early days of the cryptocurrency. However, as the Bitcoin network, cryptocurrencies and blockchain technology started becoming more and more popular, more and more people started becoming interested in mining cryptocurrencies and the difficulty of mining started increasing.
Practically speaking, it means that first simply using a regular computer was not enough and a computer had to be really powerful. Then, miners have figured out that graphic cards of computers (or graphics processing units or GPUs) could do a better job mining cryptocurrencies than the central processing units could and started building machines consisting of arrays of graphic cards or GPUs. The next step was the introduction to the market of field programmable gate arrays or FPGAs, which are integrated circuits that users can configure after the manufacturing of the devices, which is very different from central processors and regular graphics cards. Gradually, as the competition for mining Bitcoins using SHA-256 algorithm kept growing, manufacturers have introduced SHA-256 ASIC hardware designed specifically for mining bitcoins. Some of the ASIC models can perform computations up to one hundred thousand times faster compared to a regular computer central processor.