There's no difference in the loading time, if the page has many replies... maybe microseconds, but nothing more than that.
It can speed up searches, by the way.
Anyway, I thought I sent the following message to everybody, but looks like only one moderator got it. It's a faster way to delete spams, "thanks" and so on...
1 - Select one thread.
2- Click the down arrow at the top right and select split topic. Hit GO.
3 - Now, select as many postings from this thread as you wish, and click the bottom center arrow. Select "Delete Posts", hit Submit
4- Confirm your action.
This feature can be useful in those 4-years old threads with six pages of "thanks" and spams.