I've worked on a project that failed and I always felt speed was a real problem. I tried but never succeeded in convincing people that speed was the issue.

In our case, our users had a specific flow through the application they would use, and it worked, but it required clicking many (10+) buttons and waiting for a web request on each. People on the team were satisfied that the flow worked and going through it didn't take TOO long... But what people on our side didn't get is that our customers had to go through this flow dozens if not hundreds of times - some users would need to do it this many times regularly. It effectively made our users hate using the product, or they would refuse to, or they'd use it but only a little bit and they'd try to minimize the cost.

I tried to get people on our side to experience the pain points, e.g. asking PMs to follow this flow one hundred times, and things like that, but I never could get through to anyone that we should redesign and refocus on making it usable. Maybe a mockup of a faster flow was what was needed to be persuasive there.