Thank you for using Apple Support Communities.
I understand from your post that when you push the dictation button for voice to text there is a delay when recognizing that you are speaking. First, use the steps from the article below to determine if there are any issues with the microphones on your iPhone:
Get help with the microphones on your iPhone, iPad, and iPod touch - Apple Support
If the microphones are working properly, I would then restart your iPhone and continue to test for the issue.
Best Regards.