Sometimes Relevanssi can get stuck in the indexing. Relevanssi goes past the number of posts in the database and keeps indexing the posts. One possible cause is a too small max_allowed_packet value for your MySQL server.
This value controls how big queries your MySQL server can take. You can see the current value with one of these SQL commands:
SHOW VARIABLES LIKE 'max_allowed_packet'; SELECT @@global.max_allowed_packet;
A reasonable value is at least 4M (4194304). Many servers use 16M (16777216) as the default value. A value of 1M (1048576) is too small for Relevanssi and may cause indexing problems. To adjust the max_allowed_packet value, contact your service provider.