The memory leak started before they started making the trip. Someone went into the nether during that time, and you were the only one that I could find that used a portal. Regardless, I have previously tested that trip on several occasions, and it has never lagged the server out. Most likely it was a combination of the nether memory leak and the chunk loading. The trip was necessary due to the memory leak causing crashes which reset the teleporters. In any case, I happen to know that nether chunks are almost always laggier than overworld.
And for the record, those chunks have already been generated, they were merely being loaded. As I've said, I've made that trip several times.