Because this is an important problem, I would appreciate a response as soon as possible. "I would appreciate your speedy response." "I want to hear from you as soon as possible." "I look forward to hearing from you as soon as possible." (It's a little archaic.) Also, using the word "please" is acceptable in email. It shows that you are a polite person and that you are not angry with me. Finally, if you need more time, it's okay to ask for it. Say something like, "I might need a bit more time to respond since I'm still trying to figure out what to say." Or you can simply add a short note explaining why you need more time. For example: "I'll get back to you as soon as I can." That being said, don't expect a quick response if you send off an email at midnight on a Saturday.