I have an AR9 pistol. New frontier arms lower Zaviar upper I don't think it's ever jammed with Glock 17or 33 round magazines.

When using the KCI 50 round magazine it has a ridiculous amount of failure to feed.

I have been told that the reason they do not feed properly is that the bolt moves too fast for the magazine follower to come up properly. Is there a way to slow down the bolt? Is it even worth trying to mess with? Especially seeing how the 33 round magazines are a hell of a lot easier to deal with.

Any thoughts would be appreciated thanks in advance.

Frankly, for me, KCI anything is suspect and only a range toy.  If something is problematic, it gets disposed of.

Sell the drum, stick with 33s, and use the proceeds to buy a stick of chewing gum.

The reason they do not work properly is they are not designed properly. Do not modify your firearm to make it work. There is a reason Glock works because they have a reputation. KCI just wants to sell something to make money and now you are stuck with. There is probably some friction somewhere internally in the drum causing it not to feed fast enough.

Agree with above.  But what weight buffer is in that gun?

I do not know what the buffer weight is? It does rattle when you shake it though

To slow down a blowback 9, a heavier buffer is usually the prescription.  But hard to know unless we know what you have.

I have a KCI MP5 mag and my NFA C-5 jams the every other round with it and has no issues with higher dollar mags.

My PSA 9mm lower ran a KCI drum just fine, although I had to notch the left feed lip to accommodate the overinsertion tab.
I was running an 8.5 oz Spike’s ST-9X dedicated buffer.
You want to slow it down? Get a heavier buffer.
Want to know how heavy your buffer is? Get a kitchen scale.