User experience when scrolling the room list could be improved

### Steps to reproduce

1. With an account with about many rooms (about 300 in my case) scroll the room list very fast
2. Observe that the room list area is empty during the scroll

Second issue about avatars:

1. With an account with many rooms scroll the room list
2. Observe that the room avatar always takes their time (about 300 ms) to be displayed, event when they are supposed to be cached (i.e. already displayed once in the room list before)

### Outcome

#### What did you expect?

The room list to scroll and rooms to be visible, with immediate avatar rendering once supposed to be cached.

#### What happened instead?

With the fast scroll, the room list is becoming an empty area. When user stops scrolling, the rooms are displayed after a short delay.

The avatars of the room are often taking too much time to be displayed, even when not scrolling fast.

This seems to be related to the new room list.
I admit that these are not a critical issues, but I believe that fixing this will improve the overall quality feeling of the product.

Note: I have discussed this IRL with @florianduros

### Operating system

macOS

### Browser information

Firefox 147.0.2

### URL for webapp

develop.element.io

### Application version

Element version: de8ed98b53f9-js-6e3efef0c5f6 Crypto version: Rust SDK 0.16.0 (82e97d4), Vodozemac 0.9.0

### Homeserver

matrix.org

### Will you send logs?

No