I have a site on Magento CE 18.104.22.168 with 4 websites. 3 of them are 'legacy' sites which aren't really being used however it could be used in the future sometime. How much of a performance gain will I see by removing the 3 sites... 10 percent? Negligible?
I can't tell you an exact value of the performance you gain, but for sure it will be an increase in performance.
Here are some things that will take less time or memory:
- Indexing (indexing one site takes far less than 4 websites)
- all EAV selects (less data in the eav tables, less data to search through)
- global config (magento generates a config for each store view and merges all of them in one big config. fewer websites, fewer store views, smaller config)
I'm sure there are others, but this is enough to say that you will get a performance boost from removing not needed data.