Is the max transaction speed for a PoA private network the same as the main network? About 20 transactions a second?? If it is faster, would that mean the Rinkeby network is more scalable than the main network?
Rinkeby is maxed out for its given block parameters - the transaction capacity depends on
- the gas limit of each block, and
- how fast blocks are issued on the network To improve transaction capacity you need more gas per second - either by increasing 1. or decreasing 2.
Block gas limits are the maximum amount of gas allowed in a block to determine how many transactions can fit into a block. For example, let’s say we have 5 transactions where each transaction has a gas limit of 10, 20, 30, 40, and 50. If the block gas limit is 100, then the first four transactions can fit in the block.
I'm running a private PoA chain with a block gas limit of 4,700,000 running on 5 second block intervals and we can support 44 transactions per second at 21,000 gas per transaction - if we wanted we could definitely push that number higher through increasing the block gas limit or forcing all nodes to set lower transaction gas prices.
The Rinkeby network has a current gas limit of around 7,000,000 per block and roughly 15 second block intervals, which equates to 22 transactions per second at the same 21,000 gas price per transaction as in my example above. To scale up the capacity of Rinkeby the block gas limit or issuance rate would need to improve (more gas or less time).
As to whether Rinkeby is more scalable than the mainnet, you are now into the Proof of Work (PoW) vs. Proof of Authority (PoA) argument. PoA is faster but far less decentralised - you need to be 'on the list' to become a block sealer whereas in theory anybody can start mining on the PoW mainnet. While a PoA network could be scaled (and consume far less energy than an equivalent-sized PoW chain due to the lack of competitive miners all burning through GPUs trying to process the same transactions), the resulting structure of sealers would have to all know each other well enough to have voted each other in, resulting in a far more centralised chain.