Is RPI B+ capable of ffmpeg raspivid and aac audio at speed 1x?

Question

since a longtime I do livestream to youtube or twitch with 'copy' codec of raspivid (1280x720@25) and sound from /dev/zero

Now I try to add an audio track from chinese usb mic.

If I use copy audio codec, I get 'pcm_s16le' and combined with video, speed 1x and 25fps but no sounds plays from client.

I saw in twitch requirement, audio needs to be aac, so I change my ffmpeg, but as it runs, I fall to 12/13 fps and I can't reach more than 0.8 speed, too low for livestream.

/usr/bin/raspivid -o - -w 1280 -h 720 -n -t 0 -ex auto -awb auto -fps 25 -b 6000000 | \
  /usr/bin/ffmpeg -y -xerror \
  -thread_queue_size 32k -f h264 -i - \ 
  -thread_queue_size 128k -f alsa -ar 48000 -ac 1 -i hw:1,0 \ 
  -c:v copy -c:a aac -b:a 128k -strict experimental -g 50 
  -f flv /tmp/truc.flv   (or rtmp url)

at start, I get :

Input #0, h264, from 'pipe:':
  Duration: N/A, bitrate: N/A
    Stream #0:0: Video: h264 (High), yuv420p(progressive), 1280x720, 25 fps, 25 tbr, 1200k tbn, 50 tbc
Guessed Channel Layout for Input Stream #1.0 : mono
Input #1, alsa, from 'hw:1,0':
  Duration: N/A, start: 1621334643.781596, bitrate: 768 kb/s
    Stream #1:0: Audio: pcm_s16le, 48000 Hz, mono, s16, 768 kb/s
Output #0, flv, to '/tmp/truc.flv':
  Metadata:
    encoder         : Lavf57.56.101
    Stream #0:0: Video: h264 (High) ([7][0][0][0] / 0x0007), yuv420p(progressive), 1280x720, q=2-31, 25 fps, 25 tbr, 1k tbn, 1200k tbc
    Stream #0:1: Audio: aac (LC) ([10][0][0][0] / 0x000A), 48000 Hz, mono, fltp, 128 kb/s
    Metadata:
      encoder         : Lavc57.64.101 aac
Stream mapping:
  Stream #0:0 -> #0:0 (copy)
  Stream #1:0 -> #0:1 (pcm_s16le (native) -> aac (native))

Pi is running a "modest" overclock setting (800mhz) with 256Mb for GPU

model name is : ARMv6-compatible processor rev 7 (v6l)

Is this hardware is strong enough to do that ?