gizzblue is there any proof they slow the platform ?
Most of them were images -- therefore each of them would have been retrieved only once (per member) and held in cache. Redisplaying cached images has minimal impact on page retrieval or display time -- that's how web pages are intended to work and browsers are designed to do so efficiently. The slowness of the old lithium forums was primarily down to its excessively bloated software and nothing to do with signatures -- it took as long to redisplay a page as to retrieve it for the first time, so the signatures were not a cause of the delay.
A more relevant question is the impact they have on vertical height of the message. On a mobile, you really don't want signatures taking up space. On desktop, its not quite so bad and displaying them was always optional on lithium.
But perhaps the biggest short term reason against them is that there are some bugs in the signature management and some work would be needed to fix them. At the moment, there are more important ways to use the developers' time.
So for that reason, I'd suggest that signatures be put on ice for 6 months. When the dust has settled on everything else, we can possibly reconsider whether bringing them back -- maybe for desktop only, and certainly only as an option.