Why is the v3 beta slow to update with long documents?
MultiMarkdown Composer v3 has an entirely new parsing engine to improve performance. In order to apply syntax highlighting, the source text has to be parsed by MultiMarkdown to determine what structures are included (e.g. strong, emph, etc.).
The most accurate approach is to reparse the entire document every time it changes to guarantee accuracy. In longer documents, this becomes very slow.
A faster way is to reparse only the part you are editing. But it can be tricky to get this right, since some changes can actually effect a large area (turning a paragraph into a code block, for example).
The new engine in v3 is written to try to balance these two needs, but it needs testing to ensure accuracy. So the beta version actually does both -- it uses the faster approach, but then double checks by running a full reparse to ensure that the result is the same. If not, it pops an alert and asks you to send me the Composer Parser Error file so I can recreate the problem and fix it.
The final version will not run the double check, but for now the beta version will be sluggish on long documents.