Virtualdub slow?
well sure if you use whatever cccp/klite codec packs gives you as VFW encoders. Setting virtualdub to use external encoders is a bit of a hassle but you can utilize the latest version of x264, your choice of audio, and muxer. It is bound to be faster as long as it is configured properly.
Just saying. Not trying to imply converting TGA + audio to video format is a faster method than directly capturing video.
SourceNick i highly suggest using lagarith lossless encoder.
I'd rather recommend MagicYUV (https://www.magicyuv.com/) or UTvideo (http://umezawa.dyndns.info/archive/utvideo/). Lagarith is a slow lossless encoder. UTvideo is fastest of the bunch. MagicYUV apparently can record in 16bit.