Why can't a blockchain process more transactions then a single node can?

Question

I'm reading about the on-chain scalability solutions. At the moment I'm looking at sharding.

Currently, in all blockchain protocols each node stores all states (account balances, contract code and storage, etc.) and processes all transactions. This provides a large amount of security, but greatly limits scalability: a blockchain cannot process more transactions than a single node can. In large part because of this, Bitcoin is limited to ~3-7 transactions per second, Ethereum to 7-15, etc.

I cant seem to understand this, could someone elaborate?

So in other words:

The number of transactions the blockchain can process can never exceed that of a single node that is participating in the network.

Further more:

In a traditional database system, the solution to scalability is to add more servers (i.e. compute power) to handle the added transactions. In the decentralized blockchain world where every node needs to process and validate every transaction, it would require us to add more compute to every node for the network to get faster. Having no control over every public node in the network leaves us in a pickle.

So if I understand this correctly, the weakest node (or is it full node?) in the blockchain sets the limit on tx/s. Why is that? Is it possible then to add a really really slow node that slows down the whole processing of transactions?

Answer 1

Distinguish between mining nodes and non-mining nodes. The latter have nothing to do with the speed of the chain. Non-mining nodes are simply following along behind the miners.

Think now about the miners--a slow miner will almost always lose the race to produce a block to a fast miner. Therefore, they will have no ability to slow anything down. The chain simply moves forward. Slow miners keep up if they can.

Think about keeping up. A huge part of the time it takes to find a block is spent searching for the block hash at a given difficulty level. (This is why fast miners usually win--they can scan many more hashes.) Verifying the block once it's found takes very little time (by design). Once a block is found, all the nodes (both slow and fast) verify it very quickly, and everyone moves to searching for the next block hash.

The fastest miners move the chain forward. The difficulty level forces the miners to waste electricity for 14 seconds (on average) per block.

Answer 2

No, there is no limiting the Ethereum Blockchain with slow nodes. But it wouldn't survive, if the tech specs required to run a node were so high that only very powerful computers could catch up with the calculations to be done. That's why you limit Ethereum's performance, so more people can take part in the network, also with slower computers or even a Raspberry Pi (tough job though ;) ). Of course the network could agree on more performance in the next couple of years, where the average computing power per node will be higher, but do we want to leave smaller and slower devices behind?