some people have been reporting massive lag using the builders and fillers, mostly on irc but also on the forums, the latest thread : http://mod-buildcraft.com/forums/index.php?topic=1107.0

my guess for these issues, as they go away when leaving the area is that eighter the fancy particle stuff (the shooting of the red cubes or the blocks) can put a serious strain on there pc's, there might be some packet spam occuring while placing all those blocks or shooting the particles, ...

my sugestion is to add a config option or check the redindering mode so people are able to dissable said effects if they cause lag issues for them, also i think the packet stuff should be re-eveluated for 6.2 and checked for possible spams that could also create lag on not so fast computers

Some re-evaluation has been done already (and many builder packet spam bugs fixed), however - more is required.

Has this been narrowed down as either network, tick, or rendering lag yet?

@NotGyro - No. I can testify it's not networking lag, as I just did a huge optimization/rewrite of networking. It's also likely not rendering lag, as it happens even to blocks which don't do any rendering.

Most likely, this is either tick lag or a CPU/memory leak issue.

WAILA issue!