What could be the reason that my site has poor performance

The transferring of data is almost always the fastest piece, unless it's about MB's of data - in which case, bandwidth becomes an issue, so most time in a request is spent setting up TCP, SSL & waiting for the server.

The results you see could indicate issues in a number of different areas of your application such as server health, slow database queries or missed caches.

Your hosting provider could have also changed firewall/WAF or routing which would up increase TLS times, if packets are dropped & have to be resent.

There could also be large-scale outages from hosting companies affecting different regions.

Oh Dear can be configured to perform uptime checks from a location of your choice and will always perform downtime verification from a secondary location.