[BUG]: Rest Client : Isolate spawning freezes app for 1KB response

[zubaid10]

Bug Description

The REST client has a 1KB threshold for using compute() to decode JSON, which causes severe performance
issues.

Impact

• Any API response >1KB triggers isolate spawning
• Isolate spawning takes 20-87 seconds on Android devices
• UI completely freezes - users can't interact with app
• App appears to crash

Reproduction

1. Make any API call that returns >1KB response (most typical API responses)
2. Observe 20-87 second freeze on Android
3. UI is completely unresponsive during this time

Our Experience

We discovered this bug in production:

• 1.5KB response took 87 seconds to decode on Android
• Backend responded in 500ms, but frontend froze for 87 seconds
• Issue was isolate spawning (20-87s) not JSON parsing (16ms)

Root Cause

Lines 158 and 170 in rest_client_base.dart:

if (stringBody.length > 1000) {  // Too low!
  return await compute(json.decode, stringBody); // 87 second freeze
}

Solution

Increase threshold to 100KB:
if (stringBody.length > 100000) {  // 100KB is optimal
  return await compute(json.decode, stringBody);
}

Why 100KB?

- JSON parsing <100KB takes <20ms on main thread (fast!)
- Isolate spawning takes 20-87 seconds (extremely slow on Android)
- 100KB → 20ms parsing is better than 1KB → 87s isolate spawn
- See: https://github.com/flutter/flutter/issues/91606
Testing

After fix:
- 1.5KB response decoded in 16ms (was 87,000ms)
- UI never freezes
- App is responsive immediately

Recommendation

Change both thresholds from 1000 to 100000 in rest_client_base.dart

[hawkkiller]

Hey, thanks for filing this issue!

Indeed, good input. Spawning isolates for ~1000 chars is likely inefficient. However, what happens in your app is rather unexpected. Do you have the same issue when starting any isolates, even with Isolate.run?