I think you hit the bug with too low nproc limit set under the build engine. Consider upgrading to SDK 3.10 which has been released to Early Access users just a while ago - [Release notes] Sailfish SDK 3.10 [Early Access] - and which comes with this issue fixed.
Should it be not an option right now to update for you, you can raise the limits manually in your current installation:
Start the build engine either from Qt Creator or with the help of sfdk:
sfdk engine start
Enter the build engine container the raw way:
docker container exec -i -t "sailfish-sdk-build-engine_$USER" bash -i
Edit the limits
vim /etc/security/limits.d/95-nproc-sdk.conf
Replace 4096 with, e.g., 8192.
If that was really the issue, you should see the effect immediately.