Gitter sunsetting: A room with lots of hidden events is slow

[andybalaam]

Spawning from element-hq/element-meta#1339

Steps to reproduce

1. Running a local Synapse
2. Connect to the Synapse and create a room
3. Insert 10K hidden events e.g. using matrix-batch-send
4. Open the room

Outcome

What did you expect?

To be able to open the room and use it

What happened instead?

Opening the room is very slow, and when it it open, the app freezes every time I press a key in the composer.

When I try to send a message, it is very slow indeed.

Operating system

Ubuntu 22.04

Browser information

Chromium Version 109.0.5414.119 (Official Build) snap (64-bit)

URL for webapp

localhost

Application version

Element version: !!UNSET!! Olm version: 3.2.12

Homeserver

localhost

Will you send logs?

No

[andybalaam]

Chromium profile:

[andybalaam]

Profile-20230210T093649.zip

[andybalaam]

Related: element-hq/element-meta#1339

[andybalaam]

This unit test may be a good starting point for investigating this, although in exactly this form it doesn't trigger the problem:

    it("should handle large numbers of hidden events quickly", () => {
        const events = [];
        for (let i = 0; i < 10000; i++) {
            events.push(
                TestUtilsMatrix.mkEvent({
                    event: true,
                    type: "unknown.event.type",
                    content: { key: "value" },
                    room: "!room:id",
                    user: "@user:id",
                    ts: 1000000 + i,
                }),
            );
        }
        render(getComponent({ events }, { showHiddenEvents: false }));
    });

You may need to make it slightly more realistic to get into the code paths that are problematic.

[andybalaam]

I recommend looking into all the other places where we call shouldShowEvent and consider caching all of them.