If you would like to add a timer or stopwatch directly into Google Slides, one solution is to embed a YouTube video.
To do this select Insert > Video > 5 minute timer > select
- you can then adjust the dimensions of the embedded YouTube video
- you can either click on the video right away or go into Slideshow and play it then
One more item to point out is if the video you chose has audio, you can mute the audio (on the right side of the screen under format options.
Embedded below is a 1 minute and 10 second YouTube tutorial.