It seems that recently the compression level was made a configurable setting and set to the lowest compression level by default.
Why was this done?
Since content caching is done once, it seems to me using the highest level of compression (9) would be better as it would ensure smallest page sizes and save on the amount of disk/memory needed to store it.
Even in the instances when your server is sending A LOT of requests the benefit of compression (serving smaller file many time) far out weigh the extra 1/10th of a second that it would take to compress the content better.
It is possible it was set to 9, in any case when the next version of MMCache is released the compression level will default to the level of 2.4.6 and the configuration option will allow you to change it if desired.