Both the YUI compressor and Packer use scoping wisely so that variable names are substituted with “a”, “b” and “c” etc all the way down. This produces patterns that lead to more optimal gzip compression rates.
If you can’t use gzip compression for some reason, then Packer with base62 encoding turned on, is the next best thing. Scripts compressed with Packer are unpacked using one pass of a RegExp on most platforms. However, this still introduces an overhead for larger scripts. The “Decode” button on the Packer page will tell you how long the decode time is, typically 0 – 200 milliseconds. The next release of Packer will improve the decode time by about 10%. Compressors with more complex compression algorithms can have very high decode times.
Update: Check out CompressorRater!