I was following the speakers WHICH were performing that day. 和 I was following the speakers WHO were performing that day. 的差別在哪裡？如果不好說明，請提供一些例句。

No difference in meaning. However, in sentences like these - ones involving relative clauses - we use "who" for people and "which" for things, so "who" is correct in this situation. Note that you can use "that" for both people and things (as well as for pretty much any other relative clause).

✅I got my CD signed by the singer who performed at the festival.
❌I got my CD signed by the singer which performed at the festival.
✅I got my CD signed by the singer that performed at the festival.

✅This is the coat which belongs to my father.
❌This is the coat who belongs to my father.
✅This is the coat that belongs to my father.

Hope this makes it a little easier to understand!