why ripple currency exchange trading is slow sometimes? Bids Sum BTC Size BTC Bid Price XRP
  30.58995660   30.58995660 6,000.00000000
  
  Asks Ask Price XRP    Size BTC Bid Price XRP
  6,000.00  29.07743420 29.07743420


while bids and asks match above, why the orders not getting filled? The above are top entries on each side of order book for btc/xrp since last 10 minutes in the ripple client.
Nothing ever happens in Ripple unless some transaction initiates it. Precisely matching orders are permitted under some circumstances to provide enhanced liquidity. The primary purpose of order books in Ripple is to provide liquidity for payments. Having matching offers on the books permits payments in either direction with zero spread.