I did not test miner on Celeron G18xx, but know than dual core Celeron G3900 can handle 6 GPUs with 40-50% total CPU load, G1840 not much differ from G3900, 99% cpu load is abnormal. What OS and driver do you use ?
OS - win7x64SP1, drvs 391.35 and 388.43. On Pentium G3240 and 6*1070 load is about 80-85% and CPD about 0.3
Maybe speed also depends on how card is connected to mainboard? Cause i see higher speed on card connected on pci-e x8 speed without risers and slower on those connected on x1