Do you mean it's slow from outside on the Internet when going through the proxy? If that's the case, it's really easy to turn off proxying for that entry in Cloudflare and see if the problem goes away. That would be the first thing I would do.
If you mean it's slow on your internal network when Cloudflare is running then you should be setting up an internal DNS to resolve the name locally. That is, when you hit the URL it should not be giving you the outside IP address because there is no reason to do that.
When I am outside, my host name resolves like this, a non-routable internal address
bwilson@cc-TestMaps:~$ dig geoserver.wildsong.biz
...
geoserver.wildsong.biz. 300 IN A 104.21.42.186
geoserver.wildsong.biz. 300 IN A 172.67.208.22
Local:
(base) bwilson@bellman:~/.ssh$ dig geoserver.wildsong.biz
...
geoserver.wildsong.biz. 0 IN A 192.168.123.2
If you are getting a Cloudflare proxy address on your internal network then the packets for a request to your own server go out, through a Cloudflare proxy cache server, then back into your network, then back to Cloudflare then back to your client.