We found out the performance issue is connected with a scheduled maintenance in US DC datacenter:
Performance issues on TX-DC
degradation
US Texas
Admin panel
Call and video
Ticket operations
Email services
Resolved in 1 hours and 11 minutes
investigating
Performace of one of US datacenters is degraded.
update
Scheduled Network Maintenance - US-Central (Dallas)
resolved
The maintenance is over, all services work normally again