This issue might be related to a few things:
Your network connection might be slow temporarily.
Your Sketch/Photoshop artboard might be too large.
In your Sketch/Photoshop artboard, there might be some very complex vector images (like the ones imported from Illustrator or similar).
Overall, you can try dividing your Sketch/Photoshop artboards into some smaller artboards and export them again.
If this doesn't solve the issue, you can try exporting on another network to see if that helps, following our Fixing connectivity issues guide.
If you’re still experiencing issues after trying out these steps, you can contact us at support@zeplin.io.