Burning DVDs on Macbook

I bought a traditional macbook off eBay a couple of months ago and so far I love just about everything about it. The man who owned it previously did some upgrades to it, including a new DVD drive as he said the original was not burning correctly. I've since tried to burn disks and am having a problem. The burns come out fine, but they are SLOW!!!!! I can't burn above a 4.0x. On my PC, my usually range is between 11-14x. I thought it could be the media I was using, but I tried two different brands with the same results. I do a LOT of burning (close to 100 disks a week), so the slower speed is not helping. I've come up with a couple of theories: a) There's a software/preference setting I need to change, b) My drive will only burn up to 4.0x, or c) Macs can only burn up to 4.0x. Obviously a) is preferred, but anything other than c) would make me happy. Enlighten me anyone ?

100 discs a week! Why? Macs can burn at the same as PCs. Are you using cheap media?

It's probably your drive which is limited to 4x. It's definitely not a mac limitation. Can you describe what you mean by "traditional" Macbook? You might have an older macbook that can't burn faster than 4.0x to DVD.