There are two reasons you may notice compilation taking longer than
seems necessary. First, if you're compiling a 2.09 document, then
LaTeX must run in compatibility mode, which is slower than its native
LaTeX2e mode. Details on how to convert 2.09 documents to 2e
can be found in most recent LaTeX manuals. The main difference
(assuming minimal personal customizations) is that the
\documentstyle command has changed to
most packages ( .sty files) are included with the
\usepackage command, rather than as arguments to
\documentstyle. Note that arguments such as 12pt are
not packages, but are options to the class, so a straight textual
substitution is not quite sufficient.
Second, the path searching mechanism does a number of recursive directory searches. If you have a very deep directory tree (or links to one) in your personal LaTeX library, then it may take a long time to search it. Also, if it is trying to find a file which doesn't exist, then it will search the entire tree, which generally takes around ten seconds on most machines. This is frequently noticed when compiling a document for the first time, when there is no .aux file. If these few seconds are important to you, then you can short-circuit the path search by simply touch(1)ing the .aux file before running LaTeX.