Am I using bandwidth while I’m reading a thread or just while loading a thread/searching/posting?
Just while loading and searching. When you are reading a thread it is stored in your temporary internet folder.
Your name is familiar. Do you post on other message boards?
Reading does not use any bandwidth (here and on most sites, but elsewhere dynamic elements might change this.)
Loading (and reloading) uses bandwidth. The posting itself uses a little bandwidth, furthermore it causes a reload of the page. While searching uses bandwidth to display the results, the more important thing is that it causes work for the server and slows it down even if there is sufficient bandwidth.
To be more precise, any action that causes data to be sent to or from the SDMB servers is a use of bandwidth. Previewing a thread, for example. Any loading or reloading of a page, whether it’s a thread or not uses bandwidth. Searching uses bandwidth, but more to the point, it expends a huge server load. Most of the issues with the boards being slow are related to server load, not bandwidth. If anyone remembers them turning on the bullet to show threads you’ve posted in, they’ll remember how slow the boards became. But this change added zero bandwidth usage, but added a tremendous server load.
Thanks for the answers, guys. 
Two or 3 small boards (less than 200 users) regularly and a few others very occasionally.