Caldav slow and deprecated logs seen

Hi
I am on 18.0.1 and each time I modify an event by Caldav, from Thunderbird Lightning, it takes ages (around 4s) to record the modification.
In logs, I can see :

{“reqId”:“7JM6FHPhryucyfLFKw1Q”,“level”:0,“time”:“2020-03-02T20:51:49+01:00”,“remoteAddr”:“192.168.1.154”,“user”:“dominique”,“app”:“no app in context”,“method”:“PUT”,“url”:"/remote.php/dav/calendars/dominique/personal/1ddcf74b-d387-4f35-ae6c-2b80b96633f6.ics",“message”:“Deprecated event type for \OCA\DAV\CalDAV\CalDavBackend::updateCalendarObject: Symfony\Component\EventDispatcher\GenericEvent",“userAgent”:“Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0 Lightning/68.5.0”,“version”:“18.0.1.3”}
{“reqId”:“HOtx21PGJrOeNL16A35r”,“level”:0,“time”:“2020-03-02T20:51:58+01:00”,“remoteAddr”:“192.168.1.154”,“user”:“dominique”,“app”:“no app in context”,“method”:“DELETE”,“url”:”/remote.php/dav/calendars/dominique/personal/1ddcf74b-d387-4f35-ae6c-2b80b96633f6.ics",“message”:“Deprecated event type for \OCA\DAV\CalDAV\CalDavBackend::deleteCalendarObject: Symfony\Component\EventDispatcher\GenericEvent”,“userAgent”:“Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0 Lightning/68.5.0”,“version”:“18.0.1.3”}

Do you have any suggestions to retreive speed ?

Thanks a lot !
Dom