performance of phpopentracker

Hello,
i am using phpopentracker 1.5.0rc1 for some of my sites for about two month
now. 
I like its' features very much and i would like to continue using it.
So thank you for this nice piece of software.
But i have performance problems when opening my access statistics.
E.g. it takes up to two minutes when opening the samplereport. 
 
So what possibilities do i have to achieve more performance?
At the moment i have about 2 000 000 million page impressions a month.
 
Which storage engine should i use? (innodb.sql, myisam.sql,
myisam_merge.sql)
At the moment I use myisam_merge...
I think myisam should be better than innodb.sql !?
But I wonder if it was a good idea to use myisam_merge? Are temporary tables
ok for such
big amount of data?
 
Are there other possiblities to get a better performance?
 
Thanks in advance,
Tom Smets