Phone Arena has an excellent melancholic piece regarding tablets, an article that details the first attempts at actually making a slate. They date from back in the 1990s and a big part of that history is made by clunky and massive devices.
One of the older models is the GRIDPad, that was released in 1989 and it was one of the very first tablets that a person could actually afford to buy. It weighed 2 kilograms, measured 1.4 inches in thickness and it packed a 10 MHz 80C86 processor, 1 MB of RAM and a 10 inch monochrome screen with 640 x 400 pixels resolution. It also had 256 or 512 kilobyte removable storage cards and a 2400 baud modem, which took it to the dazzling speed of 2.4 kilobits per second.
It ran DOS 3.3 back then and next up we’ve got the Toshiba pen-based computer, the T100X, or Dynapad. That one weighed 1.5 kg, had 4 MB of RAM and an Intel 386 processor, clocked at 25 MHz. Its screen was a 9.5 incher monochrome unit with a 640 x 480 pixel resolution. Fujitsu also had a model introduced in 1996, the Stylistic, with an 8 inch display, a 100 MHz processor and between 8 and 40 MB of RAM.
Atari Stylus or STPad is also mentioned, being announced in 1991 and canned later on. For more details about the brilliant history of slates be sure to read all the article here.