SPIP contains its very own internal search engine. When it is enabled, a system for analysing the articles will index the contents of all your articles. This operation, even though it then offers subsequent extremely fast searches, requires a lot of work from the server hosting the site. In the case of a slow host, this may well cause a few problems.
For this reason, you can enable or disable the system indexing.
The data managed by the search engine integrated to SPIP nearly trebles the disk space used by the database. On the other hand, on slow or heavily loaded systems, indexing could cause a slight degradation of performance, or even execution time-out errors (extreme case).
Generally, if your site is very large, we advise not to use the search engine integrated to SPIP, and to choose other specialised products such as ht://Dig.
You should also note that the search engine does not index all the pages in one go. If you enable it while your site already contains a large number of articles, you will need to wait until your site records a certain number of hits (approximately) equal to the number of texts to index in order for the engine to be updated.