I noticed this too. I ran a stopwatch test:
(time differences due to two fingers, overall duration likely the same)
Golem: 8.87, 8.46, 8.64.
Skele Mage 8.37 8.44 8.52
When I spammed Shade, the health decrease was minimal.
This is either a misunderstanding of the skills or an increase of the 3%. Dread shade says “3% of its max health for each second the shade has been active”. This could be understood as 3% max health per 1 second its been active, so 6% for 2 seconds. Alternatively it could mean 3% each second until the shade is removed.