How to use grep efficiently?

Question

If you have xargs installed on a multi-core processor, you can benefit from the following just in case someone is interested.

Environment:

Processor: Dual Quad-core 2.4GHz
Memory: 32 GB
Number of files: 584450
Total Size: ~ 35 GB

Tests:

1. Find the necessary files, pipe them to xargs and tell it to execute 8 instances.

time find ./ -name "*.ext" -print0 | xargs -0 -n1 -P8 grep -H "string" >> Strings_find8

real    3m24.358s
user    1m27.654s
sys     9m40.316s

2. Find the necessary files, pipe them to xargs and tell it to execute 4 instances.

time find ./ -name "*.ext" -print0 | xargs -0 -n1 -P4 grep -H "string" >> Strings

real    16m3.051s
user    0m56.012s
sys     8m42.540s

3. Suggested by @Stephen: Find the necessary files and use + instead of xargs

time find ./ -name "*.ext" -exec grep -H "string" {} \+ >> Strings

real    53m45.438s
user    0m5.829s
sys     0m40.778s

4. Regular recursive grep.

grep -R "string" >> Strings

real    235m12.823s
user    38m57.763s
sys     38m8.301s

For my purposes, the first command worked just fine.