Strange SPDY behaviour about request time with stastic proxy cached content

Good time of day!

We use nginx as load balancer and reverse proxy for some static content
(images etc).
And a problem was found with enabling SPDY: cached content with enabled
SPDY
becomes get from server with greater request time and i think slower
(according Chrome debug console).

Here some lines from access.log, without spdy, just ssl:

request_time: “0.069” upstream_response_time: “0.069” “MISS”
request_time: “0.370” upstream_response_time: “0.211” “MISS”
request_time: “1.294” upstream_response_time: “1.200” “MISS”
and from cache:
request_time: “0.778” upstream_response_time: “-” “HIT”
request_time: “0.938” upstream_response_time: “-” “HIT”

SPDY enabled:
request_time: “0.380” upstream_response_time: “0.120” “MISS”
request_time: “1.181” upstream_response_time: “0.737” “MISS”

before now everything looks fine, but now for content from cache:
request_time: “10.389” upstream_response_time: “-” “HIT”
request_time: “9.493” upstream_response_time: “-” “HIT”

Here is a graphical illustration of problem: in the middle for some time
was
enabled spdy, but not other time on both sides.
It shows average $request_time and $upstream_response_time variables sum
every second: sum(time)/sum(requestcount) per second

Nginx cache placed on memory disk /dev/md0 , capacity 60% right now

Env: FreeBSD amd64
nginx version: nginx/1.4.2
built by gcc 4.2.2 20070831 prerelease [FreeBSD]
TLS SNI support enabled
openssl-1.0.1e

Any suggestions, thoughts, ideas?
Thanks!

Posted at Nginx Forum:
http://forum.nginx.org/read.php?2,242350,242350#msg-242350

Sorry, forgot link image for visualizing a problem:
http://i59.fastpic.ru/big/2013/0830/87/689d2b6f84dfb88fe5a57b7ad60def87.png

Posted at Nginx Forum:
http://forum.nginx.org/read.php?2,242350,242351#msg-242351