It’s pretty easy to shoot past the shield once the throw is being aimed (pretty sure this part covers the windup as well + any extra time it takes for the javelineer to aim).
I don’t agree with javelineers being good at counter-archering at all. Unless you’re facing an armada of archers, you’ll be running short spears since they’re the best of the bunch, which means you need either a HS or two hits to take down an archer. The projectile is very slow, and any decent archer will simply dodge it at medium ranges. Once you’re up close, you might as well be a maa/vang and just beat the crap out of the archer.
What javelineers absolutely excel at however is killing vanguards and (slightly less due to their faster speed and general slipperiness) MAA. They can make the life of those two classes a living hell, particularly vanguards since they are slow and don’t have any kind of defense against archers. On top of that, vanguards usually have longer weapons and stay slightly further away from your teammates, poking at them from the distance thus reducing the chance of friendly fire when you throw things at them.