Crackling and choppy recording

Have a MacBook pro from 08 with 4gb ram, 10.6.8 os. plenty of disk space. I installed Audacity and monitoring the audio sounds great. But recording is slow or something because the waveform is showing the audio is chopped off and therefor crackling. I had previously used logic pro 8 and it worked great on my computer until I had like 30 tracks in playback at once. Idk why audacity seems to require so much processing power. My recording is 16 bit btw.