I haven't noticed any long delays (more then a minute) with any US-based booking site, whether it's a US airline site or a third-party site like Expedia after submitting my credit card info. Is it possible that the delay you're experiencing be due to a slow connection and not with Mexicana's site?

Also, AFAIK as long as the connection doesn't time out, a delay in the processing time doesn't hurt anything. Just don't close the window and let the website do its thing.