In an emacs buffer when editing a file called "log/development.log" how do I quickly remove all lines containing the word "Render"
You can go to beginning of buffer with
M-x flush-lines, type your word and hit
(flush-lines REGEXP &optional RSTART REND INTERACTIVE)
Delete lines containing matches for REGEXP. When called from Lisp (and usually when called interactively as well, see below), applies to the part of the buffer after point. The line point is in is deleted if and only if it contains a match for regexp starting after point.
flush-lines also has an alias ,
delete-matching-lines which might be easier to remember for you.
You also have
delete-non-matching-lines), which deletes all of the non-matching lines, keeping only those that match.