Just a generic note on CPU usage - a single percentage number for CPU usage often doesn't tell the whole story. It depends on how many threads are in use.
Often when you see 13% usage, it's actually a single-thread application using 1 core of an 8-core CPU at 100% load (100/8 = 12.5), or 2 threads of an 16-core CPU.
One saturated thread on a quad-core CPU often shows up at 25% CPU usage.
So it depends on the application - if the conversion task is single threaded and that thread is at 100% load... it doesn't matter that your task manager is showing only 13% CPU usage, the bottleneck is in fact the CPU. Even with multi-threaded applications where thread 1 is doing X, thread 2 is doing Y and thread 3 is doing Z, if ONE of those threads is maxed out (CPU-limited) then the other threads will have to wait for it - so in effect it just takes ONE thread to stall an entire multithreaded application.
I'm not saying this is the case here (I don't do DSD), I'm just saying it's not clear cut. You need to look at the per-core CPU usage to see if one of them is maxed out. Though even if IT IS maxed out, that also doesn't necessarily mean there's a problem - it might be just converting and buffering the entire track, and will eventually drop back to 0 when it's done. If the processing is faster than realtime, then there's no clicks/stutter; otherwise, there will be stuttering as the buffer isn't filled quickly enough.