Excessive mysterious system time use in a GHC-compiled binary
Be sure to add -H128 to your build command line after
+RTS -s
Your eval looks fine, so you are good to go there.
If you really wanted to go after sluggishness on this VM, increase the thread priority on the VM (and the VM console slightly if you want).
Another unexpected penalty will be due to sync confirmation for GC (since this is SMP Debian on a multicore system).
The GC will have even more VM manipulation to perform on any multicore system, which partially explains the 61 percent GC stat and your strace and time discrepancy. The stats are not reliable for most situations anyway
You are actually doing quite well - - especially if this is on an i7 or later, for example.
I would be surprised if the -H128 option does not resolve this.
I am new here, please do let me know if I can help further or if there is anything you require prior to doling out the bounty.