In situations where a blocking redis client like redis-rb doesn't interfere with other running code (synchronous web requests), I've found zero performance issues with redis-rb (latency caused by redis is measured in single digits, usually < 5ms).
My intention wasn't to rail against redis-rb. It's written very nice, idiomatic Ruby. The faster solution was decidedly less Ruby-like. I was just suggesting that even when your profile looks to be telling you the DB is to blame, you should probably prod a bit more.
ActiveRecord is as another case where I've hit performance issues. I had just swapped out most AR usage for Sequel, so my data on that is probably well out-of-date. But, for us, materializing rows as full DAOs was often a very costly process that could dominate the cost of the DB query.
On one project my team was convinced that Redis was our bottleneck. Profiling was showing that Redis calls were where the most time was spent. I spent some time looking at the redis-rb source and it does a lot of block nesting (4 or 5 levels for every call, IIRC). So, I tried replacing the redis-rb client with something a bit lower level and our throughput doubled. It turned out that all that block processing cost as much as the network call did.
I suppose this isn't all that new: many performance issues are really a "death by 1,000 cuts" situation. Oftentimes your profiler can lead you astray.