x-sendfile seems to be a non-standard feature, supported by some servers (ie, not included for apache under debian lenny by now). Anyway, I think the problem is not how fast you transfer the data, but how often you send the file. This slice tries to send the file once, thus improving navigation speed (not transfer speed). In many occasions, this may be enough. I couldn't test if x-sendfile favors client side caching (modifying headers accordingly) Maybe this slice should be called 'smart_download' instead of 'fast_download' :-)