When loaded with too many pictures, the experiment seems to have difficulty to initialize

Hello anne235

well, the browser downloads your ~700 pictures first before starting the experiment. This may take a while.
At the moment, I have no idea why the first trial seems to run faster than the following trials. It might be due to loading the image? Hard to tell without access to the code or experiment.

If you want evenly space word and pseudoword stimuli you could use a monospaced font instead of a proportional font.

If you want evenly large stimuli on each screen your participants are using, you need an extra routine, see:

Best wishes Jens