340 bytes is not worth 20x time increase. 0.4% decrease in file size IS worth the time. If you can measure the amount of time it takes to download those last 340 bytes, then you can start to count how many downloads it takes before its worth it. (If download speed was constant, it would be about 224 downloads).
PNG seems like a good candidate for this style compression. A constant time initial cost for recurring savings in bytes transmitted seems like an excellent use of CPU time to me.
As an aside, I ran the Zopfli program with 10000 iterations on jquery-1.9.1.js:
268381 jquery-1.9.1.js
75622 jquery-1.9.1.js.gz