The CR block build time shows the accumulated time from the point when the database cache layer is notified of a CR requests until it is either sending it or needs to wait for a log flush for that block. Generally speaking, it is the time it takes to find or construct the read-consistent version of a block.
The sum of the CR build time, the flush time, and the CR send time represents the time it takes a global cache server processes to process a request for a consistent read copy of a data block made by a remote instance, in other words, the service time for a CR block. The times are measured at the cache layer and include the delays for called functions.
Essentially, the build time is mostly CPU time incurred scanning the cache for CR versions of blocks and / or creating CR copies. The process of building or finding the block should never be blocked and never require non-local memory or disk accesses. This is known as the “Light Weight Rule”.