Nginx Very slow if i use proxy pass in HTTPS via reverse proxy

Hi,
i've implemented a Reverse proxy with nginx but i've a problem, when i connect into my device via reverse proxy the device is very slow, i've tried to set the keepalive but the problem persists, the only solution is to use the protocol http in proxy pass but is not a good idea.

This is mi configuration:

location /rp/@testdevice@/ {
auth_request /_autentication_test/21222;
error_page 401 /40x.html;

resolver 127.0.0.1 valid=30s;
set $upstream_rp_21222 x.x.x.x;
proxy_pass https://$upstream_rp_21222;

#proxy_pass https://x.x.x.x/;
rewrite ^/rp/@testdevice@/(.*)$ /$1 break;
proxy_http_version 1.1;
proxy_set_header Connection "";
proxy_set_header Accept-Encoding "";
proxy_ssl_verify off;
proxy_cookie_path ~*^/.* /rp/@testdevice@/;
subs_filter_types *;
subs_filter "http://x.x.x.x/test/" "//@testdevice@/test/";
subs_filter "https://x.x.x.x:8443/" "/rp/@testdevice@/";
subs_filter "https://x.x.x.x:443/" "/rp/@testdevice@/";
subs_filter "https://x.x.x.x/" "/rp/@testdevice@/";
}


upstream xxx.xxx.xxx.xxx_443 {
server xxx.xxx.xxx.xxx:443 resolve;
keepalive 15;
keepalive_timeout 10s;
}

The nginx version is 1.25.3

Please help me :(