Perhaps you are operating with wrong parameters.
-- CPU corei7 with 4 threads
https://github.com/Thirtybird/cpuminer/releasescd C:\minerd-yacoin_stratum_x64_2_4_0
minerd-x64-corei7.exe -a scrypt-jane -t 4 -o stratum+tcp://yac.coinmine.pl:9088 -u myusername.myworker1 -p mypassword
-- GPU Radeon R7 240 4GB
https://github.com/Thirtybird/YACMiner/releasescd C:\yacminer-3.5.0-yac2-x86
setx GPU_MAX_ALLOC_PERCENT 100
setx GPU_USE_SYNC_OBJECTS 1
yacminer --scrypt-chacha --lookup-gap 6 -R 1280 -B 3560 -o stratum+tcp://yac.coinmine.pl:9088 -u myusername.myworker2 -p mypassword