I tried installing ionCube PHP Accelerator, but it conflicted with WordPress and did not work. It seemed likely to make things about 10 times faster, so I thought it was very worthwhile, but there is no point if the main content stops working.

I think it is probably not good that WordPress is throwing various errors, but I do not have time right now to deal with them one by one.

The installation itself was simple: copy a library into a path, then add an entry to php.ini.

It is a little disappointing, since installation is easy and it seemed likely to be effective.