"first, a cache should never slow you down."

Cache tangent: Of course it can and will, generally things that use caches wait for the results of those caches query before hitting whatever the slower thing is being cached. Anytime you have a cache miss and the slower thing must be queried anyway, the wasted time you spent waiting for the cache query results "slowed you down". Also applications that allocate huge caches for data sets with poor cache rates or low utilization can "slow things down" by starving other data-sets with better hit rates or higher utilization from being as large as they could be.