The speed performance of any computer is affected the amount of RAM memory and the amount of space of the primary hard drive that contains the operating system. So, check both items and upgrade both to the maximum that can be had. Back in the day, RAM was expensive so as configured by apple, iMacs came configured with minimal size. With RAM upgraded, the operating system more applications could be opened, which includes processes that run in the background. With the minimal RAM, OS 8, would hit the hard drive to swap pages of memory, rolling them in and out. Over time, the area used on hard drive would fragment, causing several read operations to assemble a page of RAM memory. This I believe was the era of using disk utility tools like Norton Disk Doctor to de-frag the hard drive. Also Tech Tools. So the easiest is to upgrade the RAM on the G3 iMac / Power Mac. Once you have upgraded the RAM, then you can then begin to upgrade the hard drive to the maximum size that your OS is capable of using. It has been so long ago I have forgotten what that value might be. I believe that the latest version of OS 8 was at revision 6 (8.6). In the latter days of the G3 power macs / iMacs, it was OS 9 (9.2). Some G3 CPUs could run OS X (10.2). The point being to upgrade RAM, hard drive space, and the Operating System. The larger capacity of the hard drive should delay the onset of fragmentation. Something to consider when upgrading the hard drive is to determine if your G3 computer is capable of using a solid state "hard drive". If you are able to do so, the speed performance is very impressive. Boot times are phenomenal. What may have taken a minute to accomplish now only takes 30 seconds. Go to www.youtube.com
enter keywords Mac G3 SSD. View several videos that others have posted about their experiences. There is a video demonstrating this guys upgrades on a G3 "Graphite" IMac, boosted to 1 gig of RAM, and a 60 gig SSD. Impressive this old Mac. Take note of MacRumors dot com website, a great resource. In general, I check YouTube videos in addition googling any particular subject. I would suggest to upgrading to "Tiger" just as he did. As it relates to your WIFI, it is possible that your WIFI card is defective or is missing. If you are running OS 9, then you have to do some investigating. If you are running OS x, then go to to the pull down menu and select "About this Mac" then select "more" the begin to see if you show an airport card. It is "iffy" whether your internal WIFI card would be able to connect with the wireless routers of today. Something to consider is to acquire an old airport base station unit and have that connected to where you currently are connected directly to your Mac. Alternately you might want to scan the forums like at MacRumors for any discussion surrounding vintage Apple products as it relates to WIFI. The limiting factor then become the graphics co processor. The OS will offload processing work to the graphics card. The greater the memory capacity of the graphics card, the faster the performance. Another website I go to is: www.macsales dot com. The have technicians you can speak with concerning upgrades. I wish you success as you seek to increase performance to the maximum. You have an excellent opportunity to do most if not all of the upgrades yourself I normally go to www.ifixit dot come. To get instructions on proper tear downs on my G5 / intel iMacs.