The MS GM Wavetable Synthesizer on XP introduces a delay of 1/10 of a second between pressing a note and hearing the sound (For Vista I was hoping that this problem was fixed but apparently it is even worse). When I first tested it on XP it was not obvious that there was a delay but I just found that I could not play advanced pieces of music in time. This is the same effect as trying to speak when listening to the sound of your own voice through headphones but with your voice delayed by a fraction of a second.
There are various possible alternatives.
There is an interesting free program called SynthFont available at http://www.synthfont.com/ which is designed to load and play soundfont sound samples. It is a windows only program. I have not tested it but I am hopeful that it will work with Piano Booster. Please report back if it works OK.
There is the Microsoft Direct Music Producer. For instructions Google for "Latency fix for the Microsoft GS Wavetable Software Synthesizer" and follow the instructions. I have tried this one out and it seems to work reasonably well. Before you can installing Microsoft Direct Music Producer you have to first run a Microsoft program to validate your have a genuine copy of windows. On a slow computer you need turn off the reverb and echo.
Timidity is another free soft syth although when I tried it on Linux it also suffered from latency problems. There is another music program/game called Synthesia and there is a thread on the forum describing how to "Using TiMidity++ to improve your Windows MIDI output". The thread is available here. https://www.synthesiagame.com/forum/viewtopic.php?f=3&t=142&start=0
The best solution is to use an external sound generator and the cheapest way is get an old Sound Blaster Live card from an old computer. I bought mine second hand from eBay very cheaply. I have just checked and the “Creative Labs Sound Blaster Live 5.1” is available for for £4.99 at ebay. I did not need the original CD as I downloaded the Creative drivers from the web. This uses the soundFont files. The Creative driver comes with a 2MB GM sound samples but there a lots of other sound font files available. The other advantage of an external sound generator is this does not load the processor and interfere with Piano Booster performance. This card also works well on Linux. You just have to type a command to load the soundfont into the card.
There is yet another program that loads the sound font files called FluidSynth, This program works very well under Linux and is now replacing Timidity as the best softsyth on Linux. On Ubuntu Studio FluidSynth ran well without any latency that I could detected. It looks like that there is some work in getting this to compile and run on windows. But I don't think any binaries or instructions for windows are available yet.