Mokwit,
Recall that IB simulates market orders for SPI and HSI. This means that IB never sends a market order for those contracts. IB instead uses an algorithm, by which it sends a limit order with a limit price generated by the algorithm, and then sends a series of cancel and replace operations to alter that limit price repeatedly, until all the size has executed. If the re-pricing algorithm has a bug, or if the algorithm is designed in such a way that it does not match the structures and behaviours of the particular vehicle or market you are trading, then it can cause very lengthy and expensive delays. It can, for example, persistently submit prices which are too conservative, so that by the time they reach the order-matching engine, the market has already run away from your order. This can happen over and over again, to the same order, while the market is rapidly moving, so that you will have price-->delay-->reprice-->delay-->reprice-->delay, etc., etc., producing a costly and outsized delay, instead of the immediate execution which would result from a better algorithm.
I have detected such problems in U.S. equity executions, and I have analyzed them in detail and persuaded IB to fix them. If you take a very detailed, blow-by-blow approach to analyzing both time and sales, and your audit trail, and get some specific info on exactly how IB reprices its simulated market orders on SPI and HSI, then you will be in a position to diagnose the problem, either on your own or with the help of IB developers, and then you can help them fix the problem so that you will be able to use market orders with confidence for SPI and HSI. The hard part is persuading the developers to communicate with you. Customer service employees are mostly incapable of helping with these problems, but there are a couple of CS guys who are exceptions and can sometimes be helpful with these problems. You might have to complain to IB management in order to get attention to your problem, or you might have to start a thread.
If market orders would help you, then you should probably consider working with IB to get this problem fixed.