[Performance] AudioStreamImporter waveform view extremely slow when maximized

### Tested versions

Reproducible in: Current `master`

### System information

Ubuntu 24.04 - Godot 4.3 Dev (`master`)

### Issue description

Ideally, moving the timeline marker around the waveform should be fast as other audio editors do it.

### Steps to reproduce

- Import any audio file that's decently big (about 2.5 - 3 mins long).

- Maximize the audio importer dialog.

- Try toggling the controls or moving the timeline marker.

### Minimal reproduction project (MRP)

[mrp.zip](https://github.com/user-attachments/files/17172909/mrp.zip)

Start with the provided MRP and reproduce the slowdown in the maximized AudioStreamImporter using a 2.5–3 minute audio file. Compare timeline-marker movement and control toggling with the importer unmaximized; done means these interactions are responsive when maximized without regressing the waveform view.

Written by the indexing model from the issue text.