Avidemux qt slow decoding

To add the subtitles to um mp4 takes 5 hours, does it also happen to you or is it just my problem?
Also uses a 100% CPU with an i7 isn’t it a bit too much?
with mkvtoolnix gui if I turn it into mkv and add the subtitle it takes maximum 5 minutes maybe even less
Thank for help

info media

Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '/run/media/don/Windows 8.1/recupero-cowon/Cocaine - La vera storia di White Boy Rick (2018).1080p.H264.english.AAC.sub.ita-iCNCrew/Cocaine - La vera storia di White Boy Rick (2018).1080p.H264.english.AAC.sub.ita-iCNCrew.mp4':
  Duration: 01:51:39.15, start: 0.000000, bitrate: 2278 kb/s
    Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 1920x784 [SAR 1:1 DAR 120:49], 2149 kb/s, 23.98 fps, 23.98 tbr, 24k tbn, 47.95 tbc (default)
    Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 124 kb/s (default)

Well, let’s start with the size of the subtitles. While I have actually never done that, size usually matters, so what exactly are we talking about?
Because you have file with the duration of about 1:51:00, I would assume it is bigger than probably 2 Gigs? Is this correct? Transcoding these files usually takes a long time. I do not know how mpeg and mkv work, but I could imagine they are different in that regard how they are storing subtitles. You could verify that by running
ffmpeg -i movie_file.mp4 movie_test_file.mp4
If that takes as long, that might be the reason.
But take my ideas with a grain of salt. I have been using ffmpeg a bit, but I am far from an expert.

Maybe I expressed myself badly.
Know that I use the google translator.
The file is a 1.8 GB mp4 without subtitles.
And so I would like to add the Italian subtitles I have in .ass
I state that with mkvtoolgui I solved in 5 minutes.
What I want to know is why avidemux takes 4 hours to insert subtitles.
Does it happen to you too?
Or is it just my problem?

Ps) the subtitle file is 125 kb

The ffmpeg commands you posted to what are they for?
I have to insert the subtitle and that’s it.
And avidemux takes too long
Thank for the help