Q: What is the difference between TDM and RTAS plug-ins?
(This response concerns Avid Pro Tools systems. The concepts however are applicable to all DAWs.)
If you have a Pro Tools HD3 system, then clearly you have a lot more money to spend than most recording enthusiasts. Or rather you did have a lot more money until you spent it!
The difference between TDM and RTAS plug-ins is that TDM plug-ins run on a card installed in your computer, RTAS plug-ins run on the computer itself.
The main advantage of TDM plug-ins is that they have their own dedicated processing resources, independent of most of the functions of the computer. RTAS plug-ins have to share processing power with whatever else the computer is doing.
TDM processing power comes in nice neat blocks, so when you have selected a plug-in for a track, you will always have enough processing power to run it. Since RTAS plug-ins share processing resources, there may be times when sufficient resources are not available. Recording or playback will stop.
With TDM plug-ins, you can easily see when all of your resources are allocated. Even when fully allocated, things will run totally smoothly. With RTAS, you're never quite sure whether you are coming up to the limit, or risk going beyond it. Not until recording or playback stops anyway.
Although there are other differences between TDM and RTAS plug-ins, they hardly make any difference in practice. If you can hear any difference in sound quality between the same plug-in in its TDM and RTAS versions, then you have the ears of a super-hero.
So if you want ease of use and can afford it, TDM is the way to go. If you can't afford it, then it will have to be RTAS.
It's hard to quantify the difference in terms of percentage. People who are light users of plug-ins won't notice the difference, so zero percent. People who use a lot of plug-ins will find TDM a tremendous relief from frustration. I would rate the benefit at 1000% or more!
By the way, there are alternative systems that use processing resources other than the computer's central processor. They can be just as good as TDM in providing certainty of resource availability.