On the Odroid-C2 I did note some messages like this:
Warning: SEQ: Giving up on finding arq 0186 in stream zone-client cache, skipping!
Warning: SEQ: Giving up on finding arq 01b1 in stream zone-client cache, skipping!
Warning: SEQ: Giving up on finding arq 01b2 in stream zone-client cache, skipping!

The fix I did was to edit /etc/sysctl.conf and add the following (sysctl -p afterwards to load)
net.core.rmem_default=524288
net.core.rmem_max=2097152
net.core.netdev_max_backlog=3000
(note: you could probably also just do real-time network thread)

====== spell cast crash
When casting upon a mob the crash happens here:
Thread 1 "showeq" received signal SIGSEGV, Segmentation fault.
strlen () at ../sysdeps/aarch64/strlen.S:92
92 ../sysdeps/aarch64/strlen.S: No such file or directory.
(gdb) bt
#0 strlen () at ../sysdeps/aarch64/strlen.S:92
#1 0x0000007fb7cf5254 in QString::fromUtf8(char const*, int) () from /usr/local/qt/lib/libqt-mt.so.3
#2 0x00000000004de128 in EQStr::formatMessage (this=0x65a3e0, formatid=<optimized out>,
arguments=0x7fffffc2f5 "\331\065+\003\nP`\270\371\340\227\363;ټC),\f\f\02 2\f\234\f\201\322Y e`\341\330\324CL\ac\216\262Ŋ", argsLen=18446744073709551608) at eqstr.cpp:165
#3 0x00000000004ea69c in MessageShell::formattedMessage (this=this@entry=0x6e2750, data=<optimized out>,
len=<optimized out>, dir=<optimized out>) at messageshell.cpp:177

To fix it: edit messageshell.cpp (add the return and it will ignore channel messages)
void MessageShell::formattedMessage(const uint8_t* data, size_t len, uint8_t dir)
{
return; // hack fix
// avoid client chatter and do nothing if not viewing channel messages
if (dir == DIR_Client)
return;

====== Opcode changes:
There appear to be some opcode changes in p1999 from the last working copy of seq. There are no mob position updates as that opcode appears to have changed. There are few others too such as this one:
Warning: OP_GroundSpawn (0x3bc2) (dataLen: 8) doesn't match: sizeof(makeDropStruct):92

====== Notes: I was more interested in checking the Odroid-C2's performance than fixing SEQ on P1999. With minor tuning the C2 appears to keep up with the network traffic and do screen updates of your character movements on the map.
I cannot check yet how it would perform at 4k monitor resolution with 100's of mob movement updates.
Perhaps with the fixes I noted to keep it from crashing someone can look at the opcode changes.