There is a compression-time, decompression-time trade-off whenever you use compression. Xz (or LZMA) is interesting in that its decode time is quite small compared to its encode time, unlike others like bz2. For large, static content the trade-off is quite good. However, with smaller files, like css and javscript sources, you really need to do some benchmarking to be sure.
On-demand xz will probably never be worth it however, the encode time just takes too long. You would have physically transferred the data by the time it has finished compressing.
On-demand xz will probably never be worth it however, the encode time just takes too long. You would have physically transferred the data by the time it has finished compressing.