Sounds like either network issues or your client is dropping frames. Are you playing on wifi or are you struggling to maintain a full 30FPS? Does it happen in every party or only when a certain player is the leader?
But I wouldn't worry too much about Gibbles in particular. For *some reason*, once any Gibbles goes into the jump animation (this includes them getting stuck trying to jump if the target is up against a wall), they're "slow" for the rest of your client's uptime. Restarting your client does fix it for a bit but it usually will happen again unless everyone in every party is super on point. Would be better to have others just use NS with their FS to flinch the Gibbles first, which makes it targetable for everyone, and then freeze it before it has a chance to move. Or if you have a cast that's trying to use a Freeze Trap on a spawn, they could use Twin Blaze to get the gifoie to flinch the Gibbles as well.