```python
import os
from moviepy.editor import *

# Check if video files exist, download if necessary
if not os.path.isfile("video.mp4"):
    os.system("youtube-dl -o video.mp4 <video_url>")
if not os.path.isfile("audio.mp3"):
    os.system("youtube-dl -o audio.mp3 <audio_url>")

# Load audio file, extract subclip, apply fade effects, analyze period
audio = AudioFileClip("audio.mp3")
subclip_audio = audio.subclip(10, 20).fadein(2).fadeout(2)
audio_period = audio.find_period()

# Load video file, extract subclip, crop, analyze looping segment
video = VideoFileClip("video.mp4")
subclip_video = video.subclip(5, 15).crop(x1=100, x2=300, y1=50, y2=250)
looping_segment = video.find_looping_segment()

# Extract looping segment, slow down, loop, create mirrored version
looping_clip = looping_segment.fx(vfx.speedx, 0.5).fx(vfx.loop, duration=subclip_audio.duration)
mirrored_clip = looping_clip.fx(vfx.mirror_x)

# Combine original and mirrored clips, apply effects, add audio
final_clip = clips_array([[looping_clip, mirrored_clip]]).fadein(2).fadeout(2).set_audio(subclip_audio)

# Create title and credits screens
title = TextClip("Title Screen", fontsize=70, color='white').set_duration(5)
credits = TextClip("Credits", fontsize=50, color='white', bg_color='black').set_duration(5)

# Concatenate title, video, credits into final video
final_video = concatenate_videoclips([title, final_clip, credits])

# Write final video to file with specified parameters
final_video.write_videofile("output.mp4", fps=30, audio_bitrate="192k", bitrate="5000k")
```