Yes, the hardcoded pure int's are only possible in the mixing rounds... giving 20% perfomance.

> although each key value is used once and only once.
once each 8 byte block... but the whole mode of operation code and cipher algo is merged all inline... so if en/decrypting more then 8 bytes each hardcoded integer of $keys is used strlen($cipertext) / 8 times.

This "high-speed-feature" is limited to 10 lambda functions.... Script-runtime wide. 
That costs, in my tests, 2MB. 

1MB RC en/decrypting takes, on my machine, with hardcoded integers inline crypting, 4.2 sec. Without it was about 5.6 sec. 

Memory will not grow unlimited. This is guaranteed. Mem costs: max. 2MB... if using RC2.php which 10 diffrent keys. If using RC2.php with 2 different keys, that 1.4 seconds will cost "only" :-) 400kb mem... 

So i think thats ok... for 20% gain in internal mode... i guess the most users are fine with this deal... hopefully. :-)