Strange SPDY behaviour about request time with stastic proxy cached content.

[endo]

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