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?
Speed boosts still apply when you're under full health, however, you're noticing a slowdown because you've entered combat mode.
Your forward running speed when not in combat is 300~ units per second. However, this slows down to 210~ units per second when you've entered combat. So the relationship here is not between your amount of health, but rather, the fact that you have entered combat mode. This decrease will also stack with other movement speed debuffs (like Chilled), though it is the only one that does so.
For the record, strafing speed is 180~ units per second, while moving backward is 105~ units per second.