Documents written for LATEX 2.09 will still be read by LATEX2e. Any such document is run in LATEX 2.09 compatibility mode.
Unfortunately, compatibility mode comes with a price: it can run up to 50% slower than LATEX 2.09 did. If you want to run your document in the faster native mode, you should try replacing the line:
\documentstyle[<options>,<packages>]{<class>}
with:
\documentclass[<options>]{<class>}
\usepackage{latexsym,<packages>}
Unfortunately, this will not always work, because some LATEX 2.09
packages will only work in LATEX2e compatibility mode. You should find
out if there is a LATEX2e version of the package available.
LATEX2e native mode also gives access to the new features of LATEX2e, described in LATEX2e for authors.