ASUROUX is your friend. you will get price improvement (quite often) - internal match is 0.18c/(1 lot of 100share) on tape a/b quite often you will not be charged - I have found this to be about 40% of the time in high liquid issues. If you're routed out, its 0.29c (per 100 lot). in liquid issues its relatively fast (faster than arca/bats/edgx/nsx maybe can be outperformed by MLDOT/SRDOT/GETCO/NIX -- again, depends on conditions)
I have not yet figured as for when it would give you zero remove and when it would charge 0.18c -- the liquidity tag it returns is the same. [anyone has an idea????]
I do etf's strictly, so for me most of the time its either route out or internal match in which case its 0.0 - since about november last year, it has been 0.0 on about 40% of the orders i route thru it, so the avg rate for me comes down to about 0.17c or so.
as all else it has its problems. in *some* issues at *some* times..it simply has NO liquidity! I have been unable so far to determine what conditions effect it. if it has no liquidity...it will route out..but by the time it does that...my train would have left already.
another problem with it, is that since november it has crashed several times -- some time it wouldnt cancel orders...(and if you have few hundreds stuck its a bit of a problem because you then find yourself suddenly filled at surprising prices :>), sometimes it simply would not except orders

this has happend several times..id say, less than 7-8.
However, even so, many people prefer it and use it as their main tool and for good reasons. In large issues, 100K+ lots are no problem too. Give it a try.
u can check ASURDOX/RDOX etc...which suppose to do the same but they aren't suppose to route you out -- therefore guaranteeing low fees (I ssay supposed, because sometimes -- they do); and they dont give price improve.
I forgot to mention QUID/QUIK...I dont use them as in the issues I trade they dont have liquidity but some people have had great success with them. the D version is dark and would give price improve *most* of the time. check them out.
Another thing that might work for you (if your profit target margins allow it ) is POSITNOW. Very expensive - 2.2$ per 1 lot (of 100 share)..but again for a reason, pretty fast good liquidity and almost never slippage. again - i dont use it, but some people trading less liquid issues with higher bid/ask spreads have had great success with it (that well deserves the fees)
Hope that helps
ps: you say u do 100k lots on big issues. how often? what is your timeframe? what is you profit target?
I do intraday with much smaller lots and quite high frequency (target is 3-5cents per trade *after* commissions and slippage);