Is it possible that the "delaying" system is "cracked"? Because 10MB/s are very few...
No. It just means you have two raid devices, md0 and md1, and it will only recover one at a time.
Your slow speed is due to slow drives, and possibly slow underlying transport.
Caching is only any good on short bursts with day to day work where you frequently read what you have just written, or it can read ahead a bit.
It doesn't help on a long read/write like a raid rebuild where every bit is read new, not cached, and written.
5400 drives are ok for light work, and better if you use multiple drives in a different raid level eg 6.
But you are finding out their limitations the hard way.
Get faster drives - min 7200. That's a third faster immediately.