[eluser]chobo[/eluser]
You can still write queries the way you use to do with php, you don't have to use the active record option CI has; I'm not even sure if that slows things down much, but makes like easier if you switch databases a lot. I suspect the extra overhead of CI won't affect your query speed, since queries would still be executed in exactly the same way as before.
Honestly, maybe I'm missing the boat on something, but I don't think using CI would really affect performance, since your "bottleneck" is just running queries on a large dataset which really doesn't have anything to do with the framework, just make sure to index your queries
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