Batch Converting 500+ Lossless Songs on Mac: Apple Silicon 8-Thread Concurrency Benchmark

Organizing an extensive digital music archive requires serious patience.

Sitting on an external USB hard drive was an archive I had maintained since college: 520 lossless tracks spanning various formats—Monkey’s Audio (.ape), uncompressed master .wav, and free lossless .flac—weighing in at 22.8 GB total.

I wanted to batch-convert the entire collection into high-quality 320 kbps MP3 files so they could fit onto a compact flash drive for my car stereo and portable media player.

Using standard single-threaded converters, transcoding 520 songs one by one takes around 3 to 4 seconds per track, meaning more than 25 minutes of thumb-twiddling. Because AConverter supports up to 8 parallel conversion tasks, I decided to run a benchmark test on my MacBook Pro to measure real-world performance under full load.

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Test Environment and Sample Dataset

• Test Machine: 14-inch MacBook Pro (Apple M3 chip, 16 GB unified memory, latest macOS).
• Source Dataset:
  • Total Tracks: 520 lossless audio files.
  • Format Breakdown: APE (160 tracks), FLAC (220 tracks), WAV (140 tracks).
  • Total Size: 22.8 GB.
• Target Output Parameters: MP3 container, constant bitrate 320 kbps, stereo channels, sample rate 44,100 Hz, preserving all ID3 tags and embedded album art.

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1-Thread vs 4-Thread vs 8-Thread Concurrency Benchmark

Inside AConverter’s settings, you can adjust the concurrency slider between 1 and 8 threads. Here are the stopwatch results across each mode:

Concurrency Mode	520 Tracks Total Time	Avg. Time Per Track	Hardware Status & Fan Acoustics
Single Thread (1 Task)	26m 14s	~3.02s / track	Efficiency cores active; 0 RPM fan silence
4 Threads Parallel	7m 18s	~0.84s / track	4 Performance cores engaged; SoC at 48°C
8 Threads Parallel (Max)	3m 42s	~0.42s / track	8 Cores fully loaded; SoC at 56°C; No fan whine

[Batch Transcoding 520 Songs - Total Duration Comparison]
Single-Thread [████████████████████████████████████████] 26m 14s
4-Threads     [██████████] 7m 18s
8-Threads     [█████] 3m 42s ── Over 700% speedup compared to single-thread!

Turning on 8-thread concurrency slashed the total runtime from nearly half an hour down to under 4 minutes!

When you drop 520 files into AConverter and hit Start, 8 progress bars spin up simultaneously, rapidly flashing green checkmarks down the list.

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System Footprint: Memory Leaks and Thermal Throttling?

A natural question when running 8 concurrent transcode workers: will it cause macOS UI lag, consume gigabytes of RAM, or spin up the fans?

Throughout the 3-minute 42-second run, I kept macOS Activity Monitor open to observe memory and CPU behavior:

1. Memory Footprint

AConverter maintained a stable RAM footprint of 65 MB to 85 MB. It employs a memory-mapped streaming buffer architecture, reading chunks from disk, encoding frames, and releasing buffers immediately. Even after crunching 22 GB of raw PCM and FLAC data, there was zero memory creep or virtual memory swapping.

2. Multi-Core Scheduling

Apple Silicon’s unified memory and asymmetric core design proved exceptional. While 8 worker threads saturated the background CPU execution units, foreground responsiveness remained flawless. Scrolling 60 FPS webpages and playing 4K videos showed zero dropped frames.

3. Thermal Behavior

The M3 processor stabilized at 56°C. The MacBook Pro’s fans never reached the audible threshold, maintaining physical silence throughout the test.

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Best Practices for Large Library Migrations

1. For Apple Silicon (M1/M2/M3/M4) Users: Set concurrency to 8 threads. It hits the sweet spot between raw throughput and energy efficiency.
2. For Older Dual-Core Intel MacBooks: Set concurrency to 2 or 4 threads to prevent thermal throttling and leave headroom for background system tasks.
3. ID3 Tag Integrity: After conversion, all 520 songs retained their original artist names, album titles, and track numbers, allowing the car audio player to catalog them immediately.

If you have a backlog of lossless albums, anime OSTs, or live bootlegs waiting to be organized, AConverter unlocks the full multi-core performance of your Mac.