Sending image is slow

### Steps to reproduce

1. Share a photo from Gallery to Element Room (self hosted)
2. Element starts uploading, says 'Waiting/Sending Message'.

### Outcome

#### What happened instead?
Element continues to say waiting and takes several minutes to upload.

Rage shake uploaded with title 'sending image is slow'

### Your phone model

_No response_

### Operating system version

_No response_

### Application version and app store

Element 1.5.7

### Homeserver

Synapse 1.71.0+focal1

### Will you send logs?

Yes

### Are you willing to provide a PR?

No