While we do our best to design Roslyn to work well for a wide range of solutions from small to very large, sometimes the defaults we choose in the balance between performance, memory usage, and useful information don't work in some solutions. In those cases, there are a number of options and in some cases registry keys that can be used to adjust things that may improve the experience.

Disabling Full Solution Diagnostics Visual Studio can analyze your entire solution in the background in order to populate the error list. This can provide a rich experience where you learn about errors before invoking Build, however, it does require significant resources. You can disable this analysis at Tools\Options\Text Editor\C#\Advanced (or Basic instead of C#). This option is available in VS 2015 RTM and later, but if you are running VS2015 Update 3 RC, the option is in the toolbar of the error list window, instead of below.
Disabling CodeLens In order to populate the Code Lens information, the IDE basically performs a Find All References operation for each method as it scrolls onto the screen. This is done in a separate process (named Microsoft.Alm.Shared.Remoting.RemoteContainer.dll), so that it won't affect Visual Studio's address space. This process also runs at Below Normal priority so as not to interfere with VS responsiveness. However, on memory constrained systems, systems with slow disk access, or on single core machines, it can still have an impact. CodeLens can be disabled at Tools\Options\Text Editor\All Languages\CodeLens, starting with VS 2013 RTM.
Adjusting File Caching Threshold We cache the syntax trees for files that have been analyzed in memory. We have the ability to serialize these files to disk to avoid running out of memory, however, there is an optimization in place to reduce disk utilization by keeping the entire contents of small files (under 4KB) in memory. Unfortunately, we've found that in some solutions with a large number of small files this can cause Visual Studio to run out of memory, since it's a 32-bit process, and therefore is limited to 4GB of memory on a 64-bit OS. The file size at which this optimization takes effect can be controlled by changing the registry. To change it, create a registry key at HKEY_CURRENT_USER\SOFTWARE\Microsoft\VisualStudio\14.0\Roslyn\Internal\Performance\Cache , and within that, create a DWORD value named "RecoverableTreeLengthThreshold" and set its value to the maximum file size (in bytes) for which syntax trees should be cached. The default value (used if the key doesn't exist) is 4096. This registry key works in VS 2015 RTM and later. (Note that some or all of the Roslyn\Internal\Performance\Cache keys may not exist and will need to be created)
Disabling Solution Crawler Even with Full Solution Analysis disabled, there are various operations that will cause Visual Studio to process all of the files in your solution. This analysis powers features like:

Populating the task list with TODO comments
Populating a cache of information for Find All References and Navigate To
Determining whether files should be opened in the designer or code view,
etc.

For our own debugging purposes, we have a registry key that can disable this. It may help relieve some issues in very large solutions. However, doing so is also likely to prevent the above features from working as designed, so this should be changed only as a last resort. To change it, create a registry key at HKEY_CURRENT_USER\SOFTWARE\Microsoft\VisualStudio\14.0\Roslyn\Internal\OnOff\Components, and within that, create a DWORD value named "Solution Crawler" and set its value to 0. This registry key works in VS 2015 RTM and later. (Note that some or all of the Roslyn\Internal\Performance\Cache keys may not exist and will need to be created)

Disabling Reference Promotion In Visual Studio 2015, we will sometimes automatically promote a "file reference" (<Reference> tag in the .csproj) to a "project to project reference" (<ProjectReference> tag in the .csproj) to give better experiences around IntelliSense and refactoring. However in some cases where there are very long chains of dependencies between projects, this can cause too much project information to be kept in memory. To disable this promotion, create a registry key at HKEY_CURRENT_USER\SOFTWARE\Microsoft\VisualStudio\14.0\Roslyn\Internal\OnOff\Features\, and within that, create a DWORD value named "Project Reference Conversion" and set its value to 0. This registry key works in VS 2015 Update 3 and later. (Note that some or all of the Roslyn\Internal\Performance\Cache keys may not exist and will need to be created)
Specifying a main method explicitly If you have a very large project that generates an executable, performance and memory usage may be improved by explicitly specifying the entry point in project properties.