README.md Driver review: improve notes on performance.

README.md

@@ -375,19 +375,30 @@ structure, pruned to contain a minimum of files, may be seen
 
 ## 1.8 Performance and hardware notes
 
+#### RAM usage
+
 The largest supported display is a 320x240 ILI9341 unit. On a Pi Pico with no
 use of frozen bytecode the demos run with about 70K of free RAM. Substantial
-improvements could be achieved using frozen bytecode.
+improvements can be achieved on all platforms by using frozen bytecode.
 
-Snappy navigation benefits from several approaches:
+#### Speed
- 1. Clocking the SPI bus as fast as possible.
- 2. Clocking the host fast (`machine.freq`).
- 3. Device driver support for `uasyncio`. Currently this exists on ILI9341 and
-    ST7789 (e.g. TTGO T-Display). I intend to extend this to other drivers.
 
 The consequence of inadequate speed is that brief button presses can be missed.
 This is because display update blocks for tens of milliseconds, during which
-time the pushbuttons are not polled. Blocking is much reduced by item 3 above.
+time the pushbuttons are not polled. This can be an issue in displays with a
+large number of pixels, multi-byte colors and/or slow SPI clock rates. In high
+resolution cases the device driver has specfic `uasyncio` support whereby the
+driver yields to the scheduler a few times during the refresh.Currently this
+exists on ILI9341 and ST7789 (e.g. TTGO T-Display). By my calculations and
+measurements this should be unnecessary on other drivers, but please report any
+tendency to miss button presses and I will investigate.
+
+This may be mitigated by two approaches:
+ 1. Clocking the SPI bus as fast as possible. This is discussed in
+ [the drivers doc](https://github.com/peterhinch/micropython-nano-gui/blob/master/DRIVERS.md).
+ 2. Clocking the host fast (`machine.freq`).
+
+#### Platform notes
 
 On the TTGO T-Display it is necessary to use physical pullup resistors on the
 pushbutton GPIO lines. This is because pins 36-39 do not have pullup support.