MER and MWD are among the toughest brokers to trade, try GS if you want a large cap broker that is liquid and a little bit easier.
The spread can be very big at "pivot" points. For example a stock hitting say 50 with size and finally he cleared all the sellers, futures breaks out, bam, he shows a BID/ASK spread of 40 cents with ASK at 50.40, chances are he won't print there next tick (unless the demand is really strong), but somewhere in between. In situations like that however unless I am really sure about the stock I won't chase it. For people already longing the stock, it is the best feeling in the world when a stock spreads it half a point, but for people who are short it is the slippage they have to pay. Yes, there can be a very long pause until the next tick, as a lot of buy orders probably queued up and the specialist is ready to blow all the stops, and then he decides where he will cross those orders. Don't worry about the time between each tick, where he fills you has nothing to do with how long your order has been in the queue, the most important factor is how many orders are before you. That said, on all but the busiest stocks at busiest times (usually some sort of news play), I get filled very quickly, and this includes GE, IBM, TYC, etc . . . It most definitely shouldn't take more than a minute. If you have Direct Plus with your broker and your order is under 1000 shares, always go limit on the offer or above the offer (when sell, hit the BID or say 10-20 cents below the BID), I have done it many times and noticed market orders can be much much slower than DP orders. Besides, say the spread is 40 cents but you think it looks really good but don't want to pay more than 15 cents spread, limit 15 cents above the BID, if you get filled, fine, if not, so what. This is critical at playing the open, as stocks can spread you half a point fills you at the top and comes right back in, limit your order and include maximum slippage yourself, don't be lazy. The only time I would use a market order is when I want to pare out some shares into strength, price improvements are very sweet, and if I get filled on an uptick with say 100 shares I am definitely holding on to rest of my position because I will know there is demand for the stock and I can probably get a better price.
This also has a lot to do with where you set your stop. Say you are short and the market looks like it is going to rip, don't wait for the stock to print above 50, once he does that, he is going to trigger all the stops there and the chance for a big spread is very high, if you think you have already lost you should either get out immediately or put your stop at 50, as 50 is typically a very liquid tick and you will get out before it blows up in your face. Of course, the risk is that if the market is weak he can hit 50 with small size then sells-off or briefly prints above 50 (blow everyone's stop), and tanks it. So it is up to you to decide whether you want to get out early or take the chance of big spread damage. With MER/MWD however, they can go as high as 50.20 to MAKE SURE that you are out before they tank the stock, and if you think "oh it is just a squeeze" before you know it you already saw 50.50 . . .
Tape reading is the key, DO NOT use a tight hard stop when you trade stocks like MER/MWD, they will blow it like no tommorrow, you have to have good discipline and just watch the tape. Seriously, there are much easier stocks than MER/MWD in XBD. I would only trade MER/MWD when the market has super momentum.