I recently upgraded my barracks from level 3 to level 4 (it stated it would be 33% quicker)....
I will list, below, the times before and after for each unit...
1 recruit / 1min / 45s / improvement = 15s/1m = 1/4 = 25% / *
1 bowman / 1min 30s / 1m / improvement = 30s/1m30s = 1/3 = 33%
1 militia / 2mins 40s / 2m / improvement = 40s/2m40s = 1/4 = 25%
1 cavalry / 6min / 4.5m / improvement = 1.5m/6m = 1/4 = 25% / *
1 longbow / 2mins 40s / 2m / improvement = 40s/2m40s = 1/4 = 25%
1 soldier / 4m / 3m /improvement = 1m/4m = 1/4 = 25% / *
1 xbow / 6m 40s / 5m / improvement = 1m40s/6m40s = 1/4 = 25%
1 elite / 10m / 7.5m / improvement = 2.5m/10m = 1/4 = 25% / *
1 cannon (not yet lvl 48 so don't know)
But as you can see, so far, only the bowmen have actually improved by 33% whereas everything else has improved by 25%. I checked with other people and it says 'improves recruting speed by 33%' and not 'up to 33%'.
Furthermore, those marked with an asterisk, are exactly divisible by 3 (original production time) and therefore can be improved by 33% (eg cannon 10 mins - 33% = 3m 20s improvement). Those not marked by an asterisk are not (but could be rounded up/down). eg. 1 xbow 6m40s - 33% = 2m13.333s improvement, but could be rounded to 2m).