Tips to speed up a Mac Mini?

I recently purchased a used MacMini3,1 (Late 2009) with a 2.26 Intel Core 2 Duo processor, and 2 GB of memory. The seller shipped it to me with Snow Leopard installed, and it ran smooth and fast. I just upgraded it to Lion and it's very sluggish. Would adding more memory fix this, or possibly upgrading to Mountain Lion make the computer run better? I installed Yosemite and it was unusably slow. I realize the computer is old, but does anyone have a solution?

The 2GB RAM is the biggest bottleneck here, no matter what you use the Mac Mini for. No recent Mac has been shipped with less than 4GB installed. Although memory management has gotten much better from Mountain Lion > Mavericks > Yosemite, you need to increase the RAM to at least 4GB.

Secondarily, you may be able retrofit a SSD to replace the slow internal HD.

I and my staff upgraded >200 Mac Minis of this vintage from 2GB to 4GB a number of years ago, in anticipation of upgrading from Snow Leopard to Lion & then eventually Mountain Lion. It enabled us to push the use of these computers another 2 years or so.

We've purchased all our RAM from DMS for more than a decade. RAM and HD/SSD retrofits can be found at a number of places, most notably Other World Computing.

(Disclaimer: We are institutionally satisfied, (and I am personally satisfied,) with both of these company's products, and have no vested or financial interest in either company.)

Opening up a Mac Mini for the first time can be a daunting operation. You're only going to want to do it once if you can help it, so plan ahead and upgrade everything at once.

Maximum RAM for that model is 8 Gb. Maximum hard drive space is a total of 2 Tb split between the two internal SATA buses (the one used by the stock hard drive, and the one used by the optical drive).

I would recommend upgrading the RAM, replacing the hard drive with the largest SSD you can afford, and seriously considering replacing the optical drive with either another SSD or a second hard drive. You can always attach an external optical drive via USB or FireWire when you need it, and optical drives are so slow compared to hard drives that the bottleneck of the USB or FireWire interface won't make much difference (whereas you'll see significantly faster data transfer speeds reading or writing large files to and from a hard disk connected to the second internal SATA bus).

For example, depending on your particular use case, you might put the operating system and applications on a small SSD, and large files you're working on frequently on a larger mechanical hard drive, both inside the case, and connect an external optical drive as needed.

Edit: Other World Computing sells the brackets and cables needed to replace the internal optical drive with a second hard drive or SSD.