The 4143 can sample at six hundred thousand samples per second, per channel. Why not use the instrument itself to measure the actual ramp rate in your actual application?... much better than a single spec based on no load and assuming conditions not applicable to your application (which likely has capacitance, lead inductance, etc. Pretty cool, right? We sure think so ;).
Once you know your ramp rate, then you can play around with the SourceAdapt functionality and optimize for your particular application, making it even faster. Isn't technology cool?