Just testing that site (thanks to maxucho for providing the example :) ). Interestingly, even if I hold my mouse button down for a second or two before releasing it, there's still a perceptible loading time of maybe 500ms - 1s.

Not sure if that's expected as part of the design?

Definitely not. May you tell me how old is your computer, and which browser/OS combination you use please? Either here or on GitHub: https://github.com/dieulot/instantclick/issues

Also please test how it goes when you press for like 5 seconds.

Hmm that could be just an issue on my site, not necessarily with the plugin. I've been experiencing some strange load times with it even before using InstantClick.