Slow load time usually comes down to a large number of screens within the prototype, large images, and/or lots of hotspots within each screen.
The number of screens per prototype, in conjunction with, the number of hotspots on each screen can slow the loading speed. Your internet speed can also have a significant impact; especially on mobile devices. From our testing, we've seen a significant difference in performance on 3G vs 4G vs Wifi.
Here are a few things to keep in mind:
We don't want to take liberties and compress your images, as clarity/quality is super important. We use the exact images you upload, which might be quite large, especially if you're doing work for retina screens.
When you create a share, we load every screen that has a hotspot linked to it so it's ready to go if someone clicks. Sometimes eliminating one hotspot that goes into a whole different part of the app/site can reduce your load time significantly.
For large prototypes:
Save images with a bit more compression to reduce overall download size, if possible. If you use PNGs, you can use something like tinypng.com.
Limit the prototype, or at least the screens within the prototype, that are linked with hotspots to 50-75.
Create share links from different starting points and load them in different browser tabs so that you can start to demo the first part as the others load.
If using Craft Sync, you can export your prototype with a pixel density of 1x. This will help reduce the overall size of your project. To learn more, check out this article: How do I set pixel density in Craft?