Very slow notification display performances in some cases

On myxwiki.org I've noticed that whenever I was going to my subwiki and clicking the bell icon, the notifications area kept spinning forever. I actually only waited 3 minutes before stopping.

see the attached video to visualize what I experiment there.

Note that myxwiki.org has a lot of subwikis and it's using the default config for the activitystream storage.

On the main DB (xwiki):

mysql> SELECT COUNT(*) FROM activitystream_events;
+----------+
| COUNT(*) |
+----------+
|   680499 |
+----------+
1 row in set (3.78 sec)

Funily if I do the same query on my subwiki DB (i.e. massol) it's not empty:

mysql> SELECT COUNT(*) FROM activitystream_events;
+----------+
| COUNT(*) |
+----------+
|    15388 |
+----------+
1 row in set (0.19 sec)