Why ExcludeFieldsFromConfigProcessor is at the end of the coveoIndexingGetFields pipeline.
This page shows the hierarchy of the coveoIndexingGetFiels pipeline: https://developers.coveo.com/display/SitecoreV4/Understanding+the+coveoIndexingGetFields+and+coveoIndexingGetTemplates+Pipelines
You can see that the ExcludeFieldsFromConfigProcessor is at the very end. Which means that it will need to process everything before excluding. This seems like a costly approach.
The reason why this pipeline is at the end is due to the processor using the final list of fields, so it needs all the other processor to be applied first. This said the less costly approach is to use the IncludeFieldsFromConfigOnlyProcessor in order to only include the fields you need. Be careful that this is a very granular approach which will require a config modification each time you add a new template to be indexed.