簡単な速度を。
さくらのVPSからabの結果です。

結果 300ms-500ms というところでした。
悪くはないですね。

あたり前ですが、さくらのネットワーク内のコンテンツとは比較にならないので。。。
やっぱり自前のAPIはさくら内に持つかフロントをすべてapp engineへ移行するかの2択のようです。
数段階のアクセスは無理なので。現状は大規模データだけ任せて細かい所は現状と同じく memcached +mysql , tokyotyrantで行くしかなさそうです。

1. 通常テキストページ

# ab -n 100 -c 20 http://gr-----.appspot.com/
Server Software:        Google
Server Hostname:       gr-----.appspot.com
Server Port:            80

Document Path:          /
Document Length:        398 bytes

Concurrency Level:      20
Time taken for tests:   1.746154 seconds
Complete requests:      100
Failed requests:        0
Write errors:           0
Total transferred:      55600 bytes
HTML transferred:       39800 bytes
Requests per second:    57.27 [#/sec] (mean)
Time per request:       349.231 [ms] (mean)
Time per request:       17.462 [ms] (mean, across all concurrent requests)
Transfer rate:          30.93 [Kbytes/sec] received

Connection Times (ms)
              min  mean[+/-sd] median   max
Connect:       40   44   4.9     43      60
Processing:   244  299  47.3    300     392
Waiting:      243  298  47.2    298     390
Total:        286  343  47.6    353     435

Percentage of the requests served within a certain time (ms)
  50%    353
  66%    366
  75%    381
  80%    394
  90%    419
  95%    421
  98%    423
  99%    435
 100%    435 (longest request)

2. datastore から get

ab -n 100 -c 20 http://gr-----.appspot.com/api/xxx?a=xxxx
Server Software:        Google
Server Hostname:        gr----.appspot.com
Server Port:            80

Document Path:          /api/xxxx?a=xxxx
Document Length:        220 bytes

Concurrency Level:      20
Time taken for tests:   1.632533 seconds
Complete requests:      100
Failed requests:        0
Write errors:           0
Total transferred:      39700 bytes
HTML transferred:       22000 bytes
Requests per second:    61.25 [#/sec] (mean)
Time per request:       326.507 [ms] (mean)
Time per request:       16.325 [ms] (mean, across all concurrent requests)
Transfer rate:          23.28 [Kbytes/sec] received

Connection Times (ms)
              min  mean[+/-sd] median   max
Connect:       40   42   2.5     42      52
Processing:   247  274  37.7    256     387
Waiting:      246  273  37.5    255     386
Total:        288  317  39.6    298     435

Percentage of the requests served within a certain time (ms)
  50%    298
  66%    306
  75%    323
  80%    347
  90%    400
  95%    409
  98%    422
  99%    435
 100%    435 (longest request)                             

3. www.google.co.jp

$ ab -n 100 -c 20 http://www.google.co.jp/
Server Software:        gws
Server Hostname:        www.google.co.jp
Server Port:            80

Document Path:          /
Document Length:        9326 bytes

Concurrency Level:      20
Time taken for tests:   0.703914 seconds
Complete requests:      100

Failed requests:        66
   (Connect: 0, Length: 66, Exceptions: 0)
Write errors:           0
Total transferred:      992608 bytes
HTML transferred:       934508 bytes
Requests per second:    142.06 [#/sec] (mean)
Time per request:       140.783 [ms] (mean)
Time per request:       7.039 [ms] (mean, across all concurrent requests)
Transfer rate:          1376.59 [Kbytes/sec] received

Connection Times (ms)
              min  mean[+/-sd] median   max
Connect:       40   43   5.2     42      64
Processing:    87   91   2.8     91     103
Waiting:       44   49   3.5     48      61
Total:        127  135   6.9    133     162

Percentage of the requests served within a certain time (ms)
  50%    133
  66%    135
  75%    138
  80%    139
  90%    145
  95%    153
  98%    157
  99%    162
 100%    162 (longest request)