The reason that find is faster on the second time is that linux does file caching. Whenever a file is accessed the first time, it keeps the contents of the file in memory (of course it only does that when you have free RAM available). If the file is read again at a later time, it can then just fetch the contents from memory without actually having to read the file again. Because memory access is much faster than disk access, this increases overall performace.
So what happens is that on the first find, most of the files aren't in memory yet, so linux has to do lots of disk operations. This is slow, so it takes some time.
When you executefind again, most of the files and directories are already in memory and it's much faster.
You can test this out yourself if you clear the cache between the two find executions. Then the second find won't be faster than the first one. Here is how it looks on my system:
# This clears the cache. Be careful through, you might loose some data (Although this shouldn't happen, it's better to be sure)$ sync && echo 3 | sudo tee /proc/sys/vm/drop_caches3$ time find /usr/lib -name "lib*"find /usr/lib/ -name "lib*" 0,47s user 1,41s system 8% cpu 21,435 total# Now the file names are in the cache. The next find is very fast:$ time find /usr/lib -name "lib*"find /usr/lib/ -name "lib*" 0,19s user 0,28s system 69% cpu 0,673 total# If we clear the cache, the time goes back to the starting time again$ sync && echo 3 | sudo tee /proc/sys/vm/drop_caches3$ time find /usr/lib -name "lib*"find /usr/lib/ -name "lib*" 0,39s user 1,45s system 10% cpu 16,866 total