If you want to multithread you need an app_config.xml in your C:\ProgramData\BOINC\Projects\www.primegrid.com

Use physical cores only (no hyperthreading) eg 2 in the example xml file below represents two cores, adjust as necessary.
For GFN21's use the following:

<app_config>
<app_version>
<app_name>genefer</app_name>
<cmdline>-nt 2</cmdline>
<avg_ncpus>2</avg_ncpus>
<plan_class>cpuGFN21</plan_class>
</app_version>
</app_config>