Document some observations regarding the latency of backends.

Closes #8.

BACKENDS.md

@@ -61,6 +61,10 @@ pipeline. In particular, choice of backend can affect:
 - **Feature set:** some FlexASIO features and options might not work with all
   backends.
 
+**Note:** the internal buffer size of the shared Windows audio pipeline has been
+observed to be 20 ms. This means that only exclusive backends (i.e. WASAPI
+Exclusive, WDM-KS) can achieve an actual latency below 20 ms.
+
 **Note:** In addition to APOs, hardware devices can also implement additional
 audio processing at a low level in the audio driver, or baked into the hardware
 itself ([DSP][]). Choice of backend cannot affect such processing.
@@ -116,12 +120,15 @@ sample rate of the input or output devices, as configured in the Windows sound
 settings. (Corollary: if the input and output devices are configured with
 different sample rates in Windows, WASAPI Shared won't work, period.) There is
 also no support for upmixing nor downmixing; the channel counts must match
-exactly. These limitations [are inherent to WASAPI itself][wasapisr].
+exactly. These limitations [are inherent to WASAPI itself][wasapisr]. It is
+reasonable to assume that this mode will provide the best possible latency for
+a shared backend.
 
 In *exclusive* mode, WASAPI behaves completely differently and bypasses the
 entirety of the Windows audio pipeline, including mixing and APOs. As a result,
 PortAudio has a direct path to the audio hardware driver, which makes for an
-ideal setup for low latency operation. The lack of APOs in the signal path can
+ideal setup for low latency operation - latencies in the single-digit
+milliseconds have been observed. The lack of APOs in the signal path can
 also be helpful in applications where fidelity to the original signal is of the
 utmost importance, and in fact, this mode can be used for "bit-perfect"
 operation. However, since mixing is bypassed, other applications cannot use the
@@ -145,6 +152,9 @@ Compared to WASAPI Exclusive, Kernel Streaming is a much older API that is
 perceived as highly complex and less reliable. WASAPI Exclusive should be
 expected to provide better results in most cases.
 
+In terms of latency, WDM-KS has been observed to perform slightly worse than
+WASAPI Exclusive, bottoming out at around 10 ms.
+
 **Note:** typically, WDM-KS will fail to initialize if the audio device is
 currently opened by any other application, even if no sound is playing.
 Consequently, it is very likely to fail when used on the default device. This

README.md

@@ -80,11 +80,9 @@ basically a fact of life and is a problem with all audio APIs and
 drivers; the only way around it is to compensate the clock dift on the
 fly using sample rate conversion, but that's much more complicated.
 
-FlexASIO has not originally been designed with latency in mind; low latency
-operation is still a work in progress. Furthermore, the default configuration
-is optimized for ease of use and reliability, not latency. That said, it should
-be possible to achieve very low latencies through careful tuning of
-configuration settings; see the example in [CONFIGURATION](CONFIGURATION.md).
+The default configuration is optimized for ease of use and reliability, not
+latency. FlexASIO is capable of achieving very low latencies when configured
+properly; see [CONFIGURATION](CONFIGURATION.md) for an example.
 
 FlexASIO is Windows-only for now. That could change in the future, as
 PortAudio itself is cross-platform.