Low latency crackle, help needed...

[quadrifoglio]

Hello everyone,

I am using Reaper for more than 7 years, and for all this time it was a brilliant experience. For me, Reaper is synonym for stability, performance, low latency, and power.
I am so delighted with this piece of software so I converted several of my friends to it, and all of them have the same experience, they work and do not think about performance, buffer, CPU, memory, and anything other than music creation.

However, few days a go another friend wanted to try Reaper and for him things look different. He has mid 2014. MacBook retina with I7 processor, and 16 gigs of RAM, Apollo Twin Thunderbolt, SSD inside, and one USB 3.0 HD for samples and projects.

The problem is crackling at low latencies, actually for anything lower than 256 samples. No matter how many channels he use, Reaper crackles while just playing few audio channels at 64 samples without any plugin and virtual instrument, while 32 samples is unusable.
Yes, I know you will say that this is too low, but actually it is not. I am using Reaper and 32 samples latency all the time with my I5 16 gigs of ram Hackintosh, and the same with my old Core2Duo I3, 4gigs of RAM laptop, and it works flawlessly. It is maybe limited to 20 or 30 plugins until it starts crackling, but at 32 samples it MUST play audio tracks without problems, which is somehow not the case here.
My Hackintosh is connected to Focusrite Saffire Pro 40 via FW, and laptop via USB 2.0 to Focusrite Scarlett 2i2, and again, everything works perfectly at 32 samples latency.
I must say, he does not want to work at 32 samples latency, we just use those figures to find out what is wrong with his setup. With this problem even 128 is to low, and just at 256 it works relatively good, but with just few channels. What would happen if he uses the full project, we do not know.
I forgot to mention that CPU usage is always at 15%, at most 25%, so it has nothing to do with computer being too slow.

We also tried with built-in audio card, and it acts the same. Then we used Logic, and at 32 samples Logic works without any problem, even with some projects with more than 30 channels, around 25 plugins and 10 virtual instruments Logic plays the projects without any crackle or click, but Reaper simply can not.
We tried to edit buffer settings in preferences, but still nothing, it crackles all the time. Then we tried to copy everything to the SSD drive, maybe it was something to do with that slower external drive, but no. It simply does not work well.

I know it is very simple, and it should work well after installing, but is there something we are doing wrong?