There's slow, then there is SLOOOOOWWWWWW. 12.6 hasn't been supported in 7 years, so shame on the customer for not following a migration to current versions. They've skipped something like 8-10 versions (12.7, all the 15 versions, and the 16 versions). IQ 12.6, 12.7, and 15.x (July 31 2016) are now out of support.
At this point they will have issues with upgrades. In place upgrades have a minimum version that we support for in place upgrades. 12.6 is too old which will drive an upgrade to be a net new build and a copy of the data not an in-place upgrade. The good news is that moving to IQ 16 will reduce their database footprint and give them much better performance. We have made just a few improvements in the 10+ years since we started IQ 12.6 development. 
I would prepare the customer for a migration to IQ 16. There are so many variables of things that may work, may not work, and certainly most are not supported.
Mark