I've just installed all three tests on a Prometeus VZSSD5 (2 core, 512 MB ram, SSD) to find out what is the performance of each forum. I'm using a standard Minstall setup, what means nginx + php-fpm + apc + mysql no-innodb.
Here are the findings (all of them stock configuration).
The command I used was siege -c100 -t 5s forum.example.com
Vanilla (github/stable)
-c100
Transactions: 122 hits
Availability: 100.00 %
Elapsed time: 4.62 secs
Data transferred: 0.25 MB
Response time: 1.94 secs
Transaction rate: 26.41 trans/sec
Throughput: 0.05 MB/sec
Concurrency: 51.26
Successful transactions: 122
Failed transactions: 0
Longest transaction: 3.53
Shortest transaction: 0.13
-c200
Transactions: 120 hits
Availability: 27.84 %
Elapsed time: 4.20 secs
Data transferred: 0.30 MB
Response time: 2.03 secs
Transaction rate: 28.57 trans/sec
Throughput: 0.07 MB/sec
Concurrency: 57.94
Successful transactions: 120
Failed transactions: 311
Longest transaction: 3.27
Shortest transaction: 0.01
MyBB
-c100
Transactions: 649 hits
Availability: 100.00 %
Elapsed time: 4.93 secs
Data transferred: 1.94 MB
Response time: 0.25 secs
Transaction rate: 131.64 trans/sec
Throughput: 0.39 MB/sec
Concurrency: 32.59
Successful transactions: 649
Failed transactions: 0
Longest transaction: 0.40
Shortest transaction: 0.03
-c200
Transactions: 673 hits
Availability: 96.70 %
Elapsed time: 4.79 secs
Data transferred: 2.01 MB
Response time: 0.73 secs
Transaction rate: 140.50 trans/sec
Throughput: 0.42 MB/sec
Concurrency: 103.14
Successful transactions: 673
Failed transactions: 23
Longest transaction: 1.00
Shortest transaction: 0.01
FluxBB
-c100
Transactions: 937 hits
Availability: 100.00 %
Elapsed time: 4.85 secs
Data transferred: 1.41 MB
Response time: 0.04 secs
Transaction rate: 193.20 trans/sec
Throughput: 0.29 MB/sec
Concurrency: 8.09
Successful transactions: 937
Failed transactions: 0
Longest transaction: 0.14
Shortest transaction: 0.01
-c200
Transactions: 1576 hits
Availability: 100.00 %
Elapsed time: 4.94 secs
Data transferred: 2.37 MB
Response time: 0.08 secs
Transaction rate: 319.03 trans/sec
Throughput: 0.48 MB/sec
Concurrency: 25.23
Successful transactions: 1576
Failed transactions: 0
Longest transaction: 0.26
Shortest transaction: 0.01
So the result is that no matter how much I'd like to use Vanilla, FluxBB has 10x the performance of Vanilla, under heavy load.
What surprises me is that how is is possible that this site uses Vanilla? Did you implement some crazy nginx microcache or Varnish solution? How do you know what pages should be cached and what should be rendered?