Those are very big trades that will face significant slippage regardless of broker.
The data I've seen (from 2004) suggests that a 5k to 10k share market order for a large cap stock sees about 7 cents (0.2%) effective spread on the NASDAQ and a 10.9 cent (0.3%) effective spread on the NYSE. I've not seen any data on larger orders, but it looks like there at least a 1.7 cent increase in the spread for each doubling of the order size. I'll be surprised if you see less than 15 cents of spread on a roundtrip for a trade of that size. Even marketable limit orders see increasing spread (and decreasing % of shares executed) for large order sizes. Spreads on mid-caps are more than twice as large due to a combination of price, float, and volume effects.
You'll also see execution issues that may effect timing and the ability to capture fast movement. The average 5k to 10k share order in large caps takes about 30 seconds for a market order, and one to two minutes on a marketable limit order. I would imagine that a 100k share order would take proportionally longer (i.e. several minutes).
You might consider breaking up your orders (especially on mid-cap stocks) and possibly even using multiple brokers to reduce slippage.
Good luck!