Element-R | count() room keys can be a slow operation on big stores, causing send performance degradation when backing up.

[BillCarsonFr]

During the backup key loop (uploading keys to the remote backup), Element-R will invoke olmMachine.roomKeyCounts().
When this operation become slow, send performaces are degraded.

Step to reproduce

Can be reproduced by typing this in the console:

window.indexedDB.open("matrix-js-sdk::matrix-sdk-crypto", 8).onsuccess = (event) => {
    const db = event.target.result;
    const start = Date.now();
    const tx = db.transaction(["inbound_group_sessions2"], "readonly");
    const object_store = tx.objectStore("inbound_group_sessions2");
    const req = object_store.count()
    req.onsuccess = (event) => { 
        console.log("## Count", event.target.result);
    };
    req.onerror  = (event) => { console.info(" ## fail to count")};
    tx.oncomplete = (() => {
        const now = Date.now();
        console.info("## Perf: count took:", now - start, "ms")
    });
}

Just after starting this, you can try to send a message. It will be blocked until the count request has completed.

The count performances are particularly bad on Firefox M1 from my testing:

Firefox:

23:07:18,566 ## Count 190003 bundle.js:262316:19
23:07:18,579 ## Perf: count took: 39182 ms

23:09:07,297 ## Count 190003 bundle.js:262316:19
23:09:07,322 ## Perf: count took: 38755 ms

There is a threshold on FF (mac M1), where the count performances drops and stop beeing O(n)

Not seen in Chrome (mac m1)

I tested on another mac (intel chip) on FF and performances stays in O(n).

This also direclty related to the size of keys in the object store (dividing the size of each room key will delay the threshold where performances drop)

[BillCarsonFr]

Related to #26783

[ara4n]

confirmed fixed for me - thanks \o/