Solr indexing takes very long when there are many objects with different values

Steps to reproduce:

1. Add 100k different users to a group.
2. Save the group
3. Profile XWiki (or just wait until Solr indexing finishes)

Expected result:

Solr indexing finishes in a minute or so.

Actual result:

Solr indexing takes a long time and 100% CPU on a core. The profile looks like this:

This profile highlights a loop whose running time is quadratic in the number of values at https://github.com/xwiki/xwiki-platform/blob/3e5272f2ef0dff06a8f4db10afd1949b2f9e6eea/xwiki-platform-core/xwiki-platform-search/xwiki-platform-search-solr/xwiki-platform-search-solr-api/src/main/java/org/xwiki/search/solr/internal/metadata/AbstractSolrMetadataExtractor.java#L513-L516