Filters and subscriptions
An event that generates a log in Ethereum is searchable with a bloom filter.
Bloom filters are space-efficient probabilistic mechanisms for identifying whether an element is likely to be within a set.
Ethereum methods use this underlying implementation to search for and retrieve specific data from the Ethereum blockchain by polling with filter methods via HTTP or listening with subscription methods via WebSocket.
Infura is even faster at this than regular Ethereum nodes as we make use of custom off-chain indexing and data storage strategies. See the following blogs for more information: