There was a research done on that a couple of years ago.
Have a look here:
One of the first things they mention is:
The average time it takes to fully load a mobile landing page is 22 seconds, according to a new analysis. Yet 53% of mobile site visitors leave a ...
You should consider a few strategies for helping users to focus their energy (attention and possibly frustration):
Use a predictive system for the length of time until an agent might help them.
This can be something you build, or a SaaS service you 'rent', or an extension of your existing software. You can outsource the work for ...
The NNG website summarizes response times and need for feedback:
Response Times: The 3 Important Limits
In summary, 0.1 seconds for the UI to feel instantaneous. From 0.1 to 1.0 seconds the UI will have a noticable delay. After 1.0 seconds the user will feel their work is being interrupted. This is independent of the technology used for the user ...
Adding onto @Nacho 's answer, I've quoted a reason why very short delays or instantaneous appearance should be avoided for tooltips:
Tooltips, expanding ads, and calls to action or information boxes that
appear over list items (such as quick-view boxes on thumbnails from
product-listing pages) are other examples of common elements with