Timwi-
Are you sure the size makes it slow? The way I understand it, it doesn't; if you split it up into several tables, it'll still have to organise those different tables.
It certainly will, but it won't have to perform, e.g., a fulltext search on the entire index, but only on the index for that table. Compare the fulltext search of the English wiki with the one on the German wiki -- that is certainly no O(log n) scalability. That's why we had to disable the English fulltext search. Now, if we merged the two into a single index, all the fulltext searches would be as slow as the English one is.
You're right, of course, that merging the tables would especially highlight unoptimized queries. But frankly, I'm not sure these need to be "highlighted" any more than they are already ;-)
Regards,
Erik