Note: edits sometimes affect multiple pages (in extreme cases, edits can affect millions of pages. The lua script (which is a wiki page editable like any other) Module:arguments is used on over 25 million pages).

There generally is a bit of a long tail effect. Popular pages get edited a lot, but they also get viewed a lot. It can be expensive when everyone is viewing and editing the same page (Micheal Jackson's death is a famous example that caused downtime, although changes were made to make things more robust so it wouldn't happen again)