This was something that Agama had mentioned before, but I never experienced it myself. Agama and Surb just tested this to make sure and they said that it does cause lag.
If you have shield or strength and someone specs you, it causes lag.
This is on a 2.4.2 server with both 2.4.2 and latest SVN clients.
Some others may want to test it also and see if they experience the same.
To recreate:
1. Get two players
2. Player1 joins a server and Player2 joins as a spectator (but Player2 is not spectating Player1 yet)
3. Player1 grabs strength or shield and runs around for a few seconds to make sure they are not choppy.
4. Player2 starts speccing Player1
5. Player1 starts experiencing choppiness
6. strength/shield expires and Player1 no longer experiences choppiness.