Do speed boosts only apply when at full health?

Or possibly, is there some other relationship between health and speed? I ask because I have some passive speed boosts: Windborne Dagger (elementalist trait passive +15% speed) and Signet of Air (+10% speed), and anytime I get damaged, I immediately slow down quite noticeably. It's a problem when I'm trying to just run through mobs without stopping to kill them. It's not like I'm crippled, I have no conditions and it happens with any enemy. The slowdown isn't as severe as a cripple or chill either. My speed comes back up when I heal. So it really feels like my passive boosts are simply ignored when I'm not at full health.

Am I correct? More importantly, is it a feature, or a bug?