I don't. I'm far from an expert on high-performance Clojure. (I'm really glad that there is such a thing, and that people focus on it, however.) Joy of Clojure and Programming Clojure get into optimizations a little bit, but I think that field is still fairly new.
Sometimes with seqs one can end up with a "holding head" problem; if you're doing stream processing but holding on to a seq, you can end up having the whole thing in memory, which would kill you. That's not what's happening there, though; a default-configured JVM can't hold anything close to a billion longs in memory.
One of the neat things is that, because the REPL actually compiles code (there's no interpreter) you get the same performance with the time macro as you would get in compiled code. What that means is that testing for performance can be done at the REPL and quickly.
To explain what I did and why, I figured that the tight loop would be optimized to Java-like performance. With the more elegant formulation, I didn't know what was going on in terms of types (how is +, a vari-aritied function with many type signatures, being handled)? If the loop performed poorly, I'd probably have put type hints on the arguments and replaced + with unchecked-add; but it performed well so I left it as it was.
Sometimes with seqs one can end up with a "holding head" problem; if you're doing stream processing but holding on to a seq, you can end up having the whole thing in memory, which would kill you. That's not what's happening there, though; a default-configured JVM can't hold anything close to a billion longs in memory.
One of the neat things is that, because the REPL actually compiles code (there's no interpreter) you get the same performance with the time macro as you would get in compiled code. What that means is that testing for performance can be done at the REPL and quickly.
To explain what I did and why, I figured that the tight loop would be optimized to Java-like performance. With the more elegant formulation, I didn't know what was going on in terms of types (how is +, a vari-aritied function with many type signatures, being handled)? If the loop performed poorly, I'd probably have put type hints on the arguments and replaced + with unchecked-add; but it performed well so I left it as it was.