Windows file usage - why does it slow down

[pblitz-github]

Win 10, Fre:ac 1.1.7

I have been taking a load of (ans in 500 to 1000 at a time!) hour-long .wav files, and converting them to .m4a. I'm running 4 threads, that seems to be able to take the CPU to 90% at maximum.

The conversion performance seems to regularly slow down and speed up again, so I've been watching both the Task Manager performance screen as well as the Resource Monitor. Initially, the input disk throughput is 15 to 10 meg/sec, with the "active time" showing at 5 to 10%. There are slight 'humps' in the active time as fre:ac reads more of the input file (as you would expect), then suddenly hits 100% - and stays there for long periods of time, slowing the disk throughput down to something like 5 to 10 meg/sec

Over in Resource Manager, the big disk read figures are by "system", reading the input wav files (the output files are smaller, so less disk activity seen). Initially, the response times are sub 10ms, but when things slow down, those figures shoot up, up to 1000ms and sometimes higher.

If I PAUSE fre:ac, then the disk activity pretty much stops, but when I un-pause it, things are slow again.

If I STOP fre:ac, then start it again, things are good again: low "active time" and low response time figures..... until it once again slows down.

Sometimes the system seems to recover (then repeat the behaviour) other times it stays slow.

While I'm here, another thing I have seen (and not quite sure how this fits with the above, if at all):

So, normally all the disk I/O is handled by "system", but after a while I start to get fre:ac reading some of the files.

Anyone have any thought / comments on the above? Any ideas what could be happening to cause the issues?

(Using Microsoft's built-in AV, if I disable it, there's no change in behaviour)

[pblitz-github]

Ok, a bit of an update: the latest conversion has been working for 90 minutes so far, and I'm noticing that, after many initial slow-downs, it is now remaining fast, with the "active time" remaining below 100% (as expected).

My experience is that, every time I start a run (typically 700+ files = 1 month of hourly log files) I get the slowdown (but when I get bored, I stop watching the performance, so not sure whether they also sped up after a while!!)