Currently there is only 1 thread for large files. On big multicore boxes scannedonlyd_clamav may be scanning with 8 or 16 threads. Having only 1 thread for big files is limiting.
Isn't it sufficient to edit line 75 in file "src/scannedonlyd_clamav.c"?
Just change "maxlargethreads" to a higher number before compilation.
this cound be made configurable indeed