Very slow "Request send" time under CloudFlare

I try to send https request to any page of my domain (even to the empty html page) with base64 code of a few megabytes file. It takes about 6-10 seconds of “Request send” time in a Chrome devtools Network stats for this query.
When i send it directly to my server IP under http in takes about 0.3seconds.
I tried to turn off any cache or security settings to make it faster but it did not work.
What should i do?

so there are 500k+ symbols of base64 encoding in POST query.
If i make a simple short query there are no issues. Any settings to fix this?

P.S. increasing of base64 text size exponentially increases the response time from server