Transactional replication, change data capture, mirroring, availability groups, log backups, full/diff backups, transaction rollbacks all read from the log.
Could be the writes to the log from the rebuild have a negative impact on read throughput
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability