I've already found this tool from 0n0e.com but the problem is that it pronounces the words too quickly. Is there any other phoneme synthesis online which speaks a bit slower?
The linked page is prominently open source. You can probably download the sources to your own computer and tweak the parameters to make it speak more slowly. It links to http://www.masswerk.at/mespeak/ which displays detailed controls for amplitude, pitch, speed, word gap, etc. The same controls are hardcoded into
mespeak.js in the fragment
$speed=175;$amp=100;$wordgap=0 so you can probably simply edit that in-place. I have not looked into how hard it is to install required dependencies etc to make it actually run on your own web server (or even, gasp, in a local file with no server).