I have noticed that the performance of the game has slowed appreciably since I did an uninstall / reinstall and loaded the patch for Baldur's Gate. When I reinstalled, I retained my save games. My system exceeds the recommnended spec. My system spec is:
CPU = AMD K6 2 300
Video Card = Voodoo 3 3000 PCI 16Mb
RAM = 64Mb
Windows 98
Hard drive = 8.2Gb
Any hints on how I can improve the game's performance?

achenar: you have VERY little RAM for BG, to be brutally honest.
Also, check to see whether the graphics options you're playing BG with are set to 16bit, 24bit or 32bit colour depth.
voodoo boards can be dreadful when it comes to colour depth performance, so you should play BG in 16bit mode. Make sure to change all graphics settings from within the game.

I have always had really good peformance from the game, with a P3-500 and only 64Mb of RAM. Perhaps it is the settings for your graphics card as Moggl suggests?

Aw Jesus I remember when I Uninstaled and then re instaled BG on my computer.It totally F**ked it up. I couldnt save any longer !!! Whenever I went to save the game cut out and went back to my desktop.So I deleated every single file on BG off my computer and had to start ALL OVER AGAIN!!!However I was only on chapter 4 at the time so I wasnt that heart broken.Anyway thats my solution to this problem.

Another thought:
Try to get the latest certified drivers for your graphics card and sound board, and try reinstalling DirectX. After that, apply the patch for BG again.