Why are packets being dropped (kfree_skb)?

I am currently sending ~9mpps, each packet is 72bytes large, and is a UDP DNS request. The packets are being sent to a Dell server I have using an intel i40e interface. The server is using all of its 32 queues and has a queue size of 4096 per queue. All client source packets have random source MAC addresses and random IP addresses in a pool of about 150. Note, that none of the network cards show dropped / discarded packets. I can see all the unicast traffic is arriving to the interface ethtool -S shows rx_unicast at ~9mpps. However rx_packets shows 1.7mpps. The egress interface shows the same amount of packets being processed and sent out the other interface I am forwarding through. Note the far end IP the generated traffic arrives at is just a L3 interface, there are no DNS services listening on it, so the packets are dropped on the far end (this is ok). When I use dropwatch I can see the following messages:

224021 drops at kfree_skb+1e (0xffffffffa33f5cbe) [software]
235277 drops at kfree_skb+1e (0xffffffffa33f5cbe) [software]
232467 drops at kfree_skb+1e (0xffffffffa33f5cbe) [software]
227083 drops at kfree_skb+1e (0xffffffffa33f5cbe) [software]
228235 drops at kfree_skb+1e (0xffffffffa33f5cbe) [software]
227216 drops at kfree_skb+1e (0xffffffffa33f5cbe) [software]
221967 drops at kfree_skb+1e (0xffffffffa33f5cbe) [software]
225418 drops at kfree_skb+1e (0xffffffffa33f5cbe) [software]

I tried looking up what 0xffffffffa33f5cbe is in /boot/System.map-$(uname -r) but nothing is listed. I tried looking up what kfree_skb does, but this it frees the sk_buff memory. (not very helpful) I believe this maybe a limitation of the kernel (forwarding, netfilter, etc.), or CPU being able to process so much traffic, however I would like to know why / and demonstrate the reason why packets are being dropped in the Kernel. How can I determine why all traffic is not being sent? **UPDATE** I was able to remove the following modules

nf_conntrack
ip_tables
iptables_filter
x_tables

This increased the speed to processing more, I can see now I am able to send ~4.5mpps, it seems that netfilter is the bottle neck. Will work more with removing other modules. Curious if anyone else have details on why this happens?