Same issue for me with checkmk cee 2.0.0p12 update with 1.3K snmp hosts & 24K services. I had to move gradually ‘fetcher helper usage’ from 13 to 200 !
Now the fetcher helper usage drops from 99% to 54% 
Meanwhile the monitoring core services check rate rises from 33 services check/sec to 340/sec 
Not a big surprise, now my checkmk VM memory is critical 
But I know well the sysadmins guys 