First of all, the load users create on a system is always depending on the type of work they do. 2 users constantly uploading assets can create more load than 100 users just typing text into pages.
Your case of working with assets is likely an intense one, so the number of users working concurrently is definitive a very important factor. 200 concurrent users working with assets ... I would not wonder in the first place that you have some bad experience regarding the performance of the UI.
In that case there are multiple paths to go:
* First of all, the newer the AEM version, the more optimizations are builtin to handle such scenarios more gracefully. 6.2 to 6.3 was a huge improvement, and also 6.3 to 6.4 gives you benefits. What AEM version are you using?
* The next step is that you need to find the bottleneck regarding performance. There are many ways to approach it, I would recommend system monitoring (mostly CPU and I/O) and also analyzing threaddumps.
CPU monitoring indicates if you should add more CPUs (or more powerful CPUs), while the threaddump analysis gives you insight where the CPU is actually spent. In many Asset instances CPU is really a limiting factor (unlike sites), so you should pay special attention to it.
But it's also good to understand where these CPU cycles are spent; it can give you hints where you should start to optimize.
HTH,
Jörg