The Solr documentation on spellchecking parameters states (emphasis mine):
If true, this parameter directs Solr to take the best suggestion for each token (if one exists) and construct a new query from the suggestions. [...]
The spellcheck.collate parameter only returns collations that are guaranteed to result in hits if re-queried, even when applying original fq parameters. This is especially helpful when there is more than one correction per query.
This only returns a query to be used. It does not actually run the suggested query.
I'd imagine in order to decide if the corrected terms yield a result Solr still has to run a variant of the original query in the background. Sure, it can ignore most parts of the original query like grouping and does not have to compute the relevance of results, but it still will have to perform the whole filter query, stemming, fuzzy search etc.
So can I expect
spellcheck.collate to have a performance impact depending on the complexity of my filter query and certain other parts of the original query?