I feel like I run slower when I activate my shield. Is this true?
It's supposed to
While I currently can't find data directly supporting this, according to this bug report on the forums, it is supposed to slow you down.
Activating your shield as Heavy Assault will slow you down. While I couldn't find any true hard data on movement speed, it is common knowledge that it slows you down when you activate it.
Here's some data on everything Heavy Assault Shield information: