"Just how much slower (wired in series) is a Williams Loco?"
Short answer: half as fast
Long answer: not any slower
In general, the speed at any given voltage will be about half of what it was before. The old top speed was about 120 scale mph at 18 volts. The new top speed will be about 60 mph at 18 volts. But... there's a minimum speed where friction exceeds the motors' torque, below which the loco will stall or run erratically. From my testing, with the motors originaly wired in parallel, the slowest you could get it to run smoothly was about 10 scale mph-- way too high for scale coupling and switching. With the motors wired in parallel, the slowest speed will still be about 8-9 mph (a slight improvement due to some complex interaction between the two motors' cogging effects and a typical power supply.) The difference is, that minimum speed will now come at a higher track voltage. So if your transformer "starts" at 6 volts you can now throttle down to a stall without cutting power to the track.
If your goal is to get the minimum speed down to 3-4 scale mph for scale switching, you'll either need a lower gear ratio (difficult or impossible in this case), or some type of closed-loop speed control. Even with speed control you'll be pushing it. These were primarily designed as toy trains, and not for precision operation. My $.02.
