Thanks, for the crashlog. I know now what is happening. This is not really a crash but iOS terminates the app because it uses too much CPU for a too long time. Memory is not an issue.
But this knowledge does not really help a lot as the only way to prevent this would be to throttle down the import speed by adding a bit of cooldown time between each song where the CPU would do nothing. But that would increase the import times.
I will have to do some reading if there is a way to ensure that the cpu usage stays below the threshold that iOS is monitoring without impacting the import speed too much.