Multi-core scalability problems

Hello,
I'm testing the performance of XDP when dropping packets using multiple 
cores and I'm getting unexpected results.
My machine is equipped with a dual port Intel XL710 40 GbE and an Intel 
Xeon Gold 5120 CPU @ 2.20GHz with 14 cores (HyperThreading disabled), 
running Ubuntu server 18.04 with kernel 5.8.12.
I'm using the xdp_rxq_info program from the kernel tree samples to drop 
packets.
I generate 64 bytes UDP packets with MoonGen for a total of 42 Mpps. 
Packets are uniformly distributed in different flows (different src 
port) and I use flow direction rules on the rx NIC to send these flows 
to different queues/cores.
Here are my results:

1 FLOW:
Running XDP on dev:enp101s0f0 (ifindex:3) action:XDP_DROP options:no_touch
XDP stats       CPU     pps         issue-pps
XDP-RX CPU      0       17784270    0
XDP-RX CPU      total   17784270

RXQ stats       RXQ:CPU pps         issue-pps
rx_queue_index    0:0   17784270    0
rx_queue_index    0:sum 17784270
---

2 FLOWS:
Running XDP on dev:enp101s0f0 (ifindex:3) action:XDP_DROP options:no_touch
XDP stats       CPU     pps         issue-pps
XDP-RX CPU      0       7016363     0
XDP-RX CPU      1       7017291     0
XDP-RX CPU      total   14033655

RXQ stats       RXQ:CPU pps         issue-pps
rx_queue_index    0:0   7016366     0
rx_queue_index    0:sum 7016366
rx_queue_index    1:1   7017294     0
rx_queue_index    1:sum 7017294
---

4 FLOWS:
Running XDP on dev:enp101s0f0 (ifindex:3) action:XDP_DROP options:no_touch
XDP stats       CPU     pps         issue-pps
XDP-RX CPU      0       2359478     0
XDP-RX CPU      1       2358508     0
XDP-RX CPU      2       2357042     0
XDP-RX CPU      3       2355396     0
XDP-RX CPU      total   9430425

RXQ stats       RXQ:CPU pps         issue-pps
rx_queue_index    0:0   2359474     0
rx_queue_index    0:sum 2359474
rx_queue_index    1:1   2358504     0
rx_queue_index    1:sum 2358504
rx_queue_index    2:2   2357040     0
rx_queue_index    2:sum 2357040
rx_queue_index    3:3   2355392     0
rx_queue_index    3:sum 2355392

I don't understand why overall performance is reducing with the number 
of cores, according to [1] I would expect it to increase until reaching 
a maximum value. Is there any parameter I should tune to overcome the 
problem?

Thanks in advance for your help.
Federico Parola

[1] 
https://github.com/tohojo/xdp-paper/blob/master/benchmarks/bench02_xdp_drop.org