When your computer has a multi-core processor (or multiple processors), VeraCrypt uses all of the cores (or processors) in parallel for encryption and decryption. For example, when VeraCrypt is to decrypt a chunk of data, it first splits the chunk into several
smaller pieces. The number of the pieces is equal to the number of the cores (or processors). Then, all of the pieces are decrypted in parallel (piece 1 is decrypted by thread 1, piece 2 is decrypted by thread 2, etc). The same method is used for encryption.
So if your computer has, for example, a quad-core processor, then encryption and decryption are four times faster than on a single-core processor with equivalent specifications (likewise, they are twice faster on dual-core processors, etc).
Increase in encryption/decryption speed is directly proportional to the number of cores and/or processors.
Note: Processors with the Hyper-Threading technology provide multiple logical cores per one physical core (or multiple logical processors per one physical processor). When Hyper Threading is enabled in the computer firmware (e.g. BIOS) settings, VeraCrypt creates
one thread for each logical core/processor. For example, on a 6-core processor that provides two logical cores per one physical core, VeraCrypt uses 12 threads.
When your computer has a multi-core processor/CPU (or multiple processors/CPUs),
header key derivation is parallelized too. As a result, mounting of a volume is several times faster on a multi-core processor (or multi-processor computer) than on a single-core processor (or a single-processor computer) with equivalent specifications.