cost of syscall very different on similar machines

I have two machines:

1) 2 x E5-2620 v3 @ 2.40GHz (microcode: 0x3c 2018-01-19), 8GB RAM (CentOS 7 w/ 4.11 kernel)

2) 2 x E5-2630 v3 @ 2.40GHz (microcode: 0x3c 2018-01-19), 64GB RAM (Fedora 22 w/ 4.11 kernel)

The kernels are custom compiled but otherwise without any modification. The .config for both builds are exactly the same (and neither have kpti since it was introduce on 4.14 iirc).

I wrote a code like:

beg = rdtsc_beg();
for (i = 0; i < 1000000; ++i)
{
    syscall(512); //ENOSYS
}
end = rdtsc_end();

printf("syscall: %lu cycles\n", (end - beg) / 1000000);

to measure the cost of a syscall. I get 99 cycles on machine 1 and 264 cycles on machine 2.

I can not understand why there is such a huge difference given almost anything that I think is relevant being the same.

Any ideas what might be causing the difference or any clue where I should be looking to find out the cause?

Edit:

I changed the code to:

volatile int ret = 0;

beg = rdtsc_beg();
for (i = 0; i < 1000000; ++i)
{
    ret++;
}
end = rdtsc_end();

printf("inc: %lu cycles\n", (end - beg) / 1000000);

And compiled statically:

400a3f:       0f a2                   cpuid
400a41:       0f 31                   rdtsc
400a43:       89 d6                   mov    %edx,%esi
400a45:       89 c7                   mov    %eax,%edi
400a47:       48 c1 e6 20             shl    $0x20,%rsi
400a4b:       89 ff                   mov    %edi,%edi
400a4d:       b8 80 96 98 00          mov    $0xf4240,%eax
400a52:       48 09 fe                or     %rdi,%rsi
400a55:       0f 1f 00                nopl   (%rax)
400a58:       8b 54 24 0c             mov    0xc(%rsp),%edx
400a5c:       83 c2 01                add    $0x1,%edx
400a5f:       48 83 e8 01             sub    $0x1,%rax
400a63:       89 54 24 0c             mov    %edx,0xc(%rsp)
400a67:       75 ef                   jne    400a58 <main+0x28>
400a69:       0f 01 f9                rdtscp
400a6c:       41 89 d0                mov    %edx,%r8d
400a6f:       89 c7                   mov    %eax,%edi
400a71:       0f a2                   cpuid

The result is 5 cycles on Machine 1 vs 14 cycles in Machine 2.