Can you please elaborate on how you are producing the issue you described?
While running the "C# Editor (.NET Lang. Add-on)" sample, the sample file is relatively small and there is not a very big comment block in the sample. I zoomed out to the minimum level where pretty much the entire document was visible, so there wasn't much to scroll. I then commented out the entire file, copied it, and then pasted it several times to produce about 1500 lines of code, all comments. I was able to use the mouse wheel to scroll up and down without any noticeable delay in scrolling.
I also used our "SDI Code Editor (in-line)" sample with the same 1500-line file of comments and used both the "C# - Free Language Definition" and "C# (.NET Languages Add-on)" languages. I did not observe any scrolling issues. I also tried zooming in all the way (in case I misunderstood which direction you zoomed) and still did not observe any scrolling issues.