What is the fastest way to generate video with subtitles on node.js + ffmpeg?

Question

fmpeg

Introductory: Pieces
of text fly from the user from the site
Pieces arrive at the node.js server
At the output, you need to get a video glued from all pieces of text, voiced by a machine voice, with added subtitles and an audio background. So that it can be rummaged in the social. networks (vk does not accept the .mkv format)
Options that we tried:
1. Get all the text at once, generate all the speech, generate a file with subtitles, burn subtitles into the video (vk does not support the .mkv container) and give it away. It took 12 seconds of operations for a 45-second video on a local computer.
2. For each piece of text generate audio and video files (with added subtitles). It took 1 second for one piece for one client .On the final request, only glue everything and give back the glued. The last request took 2-3 seconds, which is already tolerable.
Option 2 looks acceptable in terms of speed, but if you run 50 clients simultaneously, then the computer (tested on a MacBook PRO 2013, 2.4 GHz i7, 8gb 1600 Mhz DDR3, SSD 256gb) processed only 1 piece from 1 client in 60 seconds (60 times slower), then the computer hung tightly.
Audio generation done via Amazon Polly
Subtitle generation via https://github.com/gsantiago/subtitle.js#readme
Call ffmpeg from node.js via https://github.com/fluent-ffmpeg/node-fluent-ffmpeg
video - 1280x720
Used commands:

ffmpeg -i import/back.mov -i export_0/tmp.srt -scodec mov_text -t 6 export_0/output.mov

- burning video with subtitles and trimming to conditional 6 seconds (we send unix timestamp in the code)

ffmpeg -i audio1.mp3  .... -i audio15.mp3  merged.mp3

- merging all audio

ffmpeg -i merged.mp3  -i back.mp3 -filter_complex amerge -ac 2-c:a libmp3lame -q:a 4 -shortest audio.mp3

- overlay audio background on the text
ffmpeg -i video.txt -f concat -c copy video.mp4- gluing all videos

ffmpeg -i audio.mp3 -i video.mp4 -i test.mp4 -i export/output.mp3 -c:v copy -c:a aac -map 0:v:0 -map 1:a:0 -shortest output.mp4

- overlaying audio on video
Questions that torment:
1. Is it possible faster?
2. Can you use other codecs or gluing methods without recoding?
3. Try to call ffmpeg commands directly without a wrapper? (actually gives 50-100ms of speed)
4. Try not to save to disk, but write data to Stream and glue them together in the end?

Answer 1

1. you can: use codecs in intermediate files that allow contact
   They can then be concatenated quickly-i "concat:video1.ts|video2.ts"
   
2. it's probably faster to run the shell script once with all the ffmpeg commands
   
3. disk is usually fast enough