'yslow' is an addition to the FireBug firefox extention which analyse and give the ration and resource link on YahooDevNetwork
http://developer.yahoo.com/yslow/

if you check your Drupal site with JStools module installed you will get the 'Performance Grade' of your site and one of the factor will be the jsTools with

'The following JavaScript files do not appear to be obfuscated nor minified'
for all .js files of the module

i would suggest, -or rather YahooDev suggests the inclusion of the minifyJS for faster performance
http://developer.yahoo.com/performance/rules.html#minify

thanks