JET

Please excuse my bad English. I tried pixel brush tool and I found out when I try to draw quickly its reaction gets slower. For example, ■■■■■■■■■■■■■■■■■■■■■■ This is what I want to get, ■■■■■■■■■■■■■■■■ But this appears. At first time I thought it was a problem on my pen tablet or its driver. But I found out same thing happens with laptop touchpad! What should I do? My Mac OS version is OS X Yosemite 10.10.2.