What's the difference between indexed and non-indexed field in Views and Search API?
Simple:

Title provided for Views from Content entity.
Title (indexed field) provided for Views from SOLR.

For example: You have 1000 contents, and you want to show 10 of these, titles of that starts &quot;Z&quot; letter.
If you use Title as filter - Drupal will use it by Views handlers, so:

It will request data from SOLR server, in our case - all 1000 items.
The Views filter on found items, reducing it to the required 10 &quot;Z&quot; contents.

As you see - it's not good for performance. Better is using Title (indexed field)as Views filter, that will request only required 10 records from SOLR server.
p.s. Against it, title will prefer for Views output than indexed field (for some cases).