Final answer:
Bitcoin transaction speeds can be slow primarily due to high network congestion as the blockchain has a limit on the number of transactions per block, while decreased miner activity can also contribute to delays.
Step-by-step explanation:
The speed of Bitcoin transactions can sometimes be slow, and the primary reason for this is b) High network congestion. When too many transactions are submitted to the Bitcoin network at once, it causes delays because there is a limit to how many transactions can be included in each block of the blockchain. The miners who add transactions to the blocks prioritize those with higher fees, which means that transactions with lower fees may be delayed during times of high congestion.
Other factors that can affect the speed of Bitcoin transactions are d) Decreased miner activity, where fewer miners are available to process transactions, and the capacity of the block in terms of its size, which determines how many transactions can be included in each block. However, a small blockchain size is not an issue since the individual blocks have a set capacity.