adjust some timings in schema.md

The new numbers are taken from my odroid setup. In particular, the size check
is noticeably slower than what I'd gathered before, enough to show that it
shouldn't be performed on startup.

design/schema.md

@@ -335,8 +335,8 @@ The times are roughly:
 
 | level    | operation   |     time |
 | :------- | :---------- | -------: |
-| presence | `readdir()` |   ~3 sec |
-| size     | `fstat()`   |   ~3 sec |
+| presence | `readdir()` | ~1.6 sec |
+| size     | `fstat()`   | ~100 sec |
 | hash     | `read()`    | ~8 hours |
 
 The `readdir()` and `fstat()` times can be tested simply:
@@ -345,9 +345,9 @@ The `readdir()` and `fstat()` times can be tested simply:
     $ cd testdir
     $ seq 1 $[60*24*365*6/12*2] | xargs touch
     $ sudo sh -c 'echo 1 > /proc/sys/vm/drop_caches'
-    $ time ls -1 -F | wc -l
+    $ time ls -1 -f | wc -l
     $ sudo sh -c 'echo 1 > /proc/sys/vm/drop_caches'
-    $ time ls -1 -F --size | wc -l
+    $ time ls -1 -f --size | wc -l
 
     (The system calls used by `ls` can be verified through strace.)
 
@@ -358,10 +358,11 @@ the Raspberry Pi 2, flash, network, and disk are all on the same USB 2.0 bus
 to be about 25 MB/sec on an idle system (~40% of the theoretical 480
 Mbit/sec). Therefore the process will take over a day.
 
-The size check is fast enough that it seems reasonable to simply always
-perform it on startup. Hash checks are too expensive to wait for in normal
-operation; they will either be a rare offline data recovery mechanism or done
-in the background at low priority.
+The presence check is fast enough that it seems reasonable to simply always
+perform it on startup. Size could be checked with a verification command used
+for more extensive verification, such as before and after schema upgrades.
+Hash checks could be performed in a rare offline data recovery mechanism or in
+the background at low priority.
 
 ### Recording table