Desktop frozen for a few minutes on Ubuntu 20.04

The desktop froze during a few minutes: the menus were responsive but the windows did not allow any interaction. It came back to normal after a few minutes. The only relevant line in syslog is

[mar. janv.  5 12:02:42 2021] perf: interrupt took too long (3136 > 3130), lowering kernel.perf_event_max_sample_rate to 63750

and nothing in the 15 minutes before that.