Extreme brush lag when onion skin is enabled?

Onion skin is a huge part in helping me animate, but whenever I have it on, the more frames I have drawn in, it gets laggier and laggier. It becomes virtually impossible to draw, sometimes it doesn’t even register any contact with the screen, sometime it obscures the line completely. Though when I turn off onion skin, its all fine and dandy.
How can I prevent lag or help reduce it when onion skin is on? I /really/ need it to work smoothly.

You left out the graphic card which is probably the most relevant hardware.

What are the details of your system?

Windows 7 Professional

Rating: 5.9
Processor: Intel(R) Core™ i5-2520M CPU @ 2.50GHz 2.50 GHz
Installed Memory (RAM): 8.00 GB (7.89 GB usable)
System type: 64-bit Operating System
Pen and Touch: Pen and Touch Input Available with 2 Touch Points

Just a side note, I’ve also been getting a lot of lag when I play the openGL frames, like hit enter to play them. It sometimes “skips a beat” per se, or plays the frames very eradically(as I’m watching the dial being read), especially after frame 50-60(?) on a certain project I’ve been working on. The brush is so laggy that I’ve predrawn them in Sai and imported them into the stage than actually drawing them inside toonboom–do having lots of bitmap images slow the software down, or is it just my system? Do I need to bulk up on some processor stuff?

Vector is better as bitmap does tax everything.