I was able to run small maps (256x256 or 128x256) on a 1.2 GHz android phone with 1 GB of ram (Dual-core 1.2 GHz Cortex-A9). When the map filled up with ~60 or so trains, it became too slow to play comfortably. The fast-forward feature only sped things up by approximately 1.5x at the start of the game, and barely any effect later in the game.
Now I'm running on a 2.4 GHz android (Snapdragon 801 Quad-core). Still running smaller maps because anything large in inconvenient for thumb operation, but the speedup feature works super fast (3-5 times faster), and haven't run into any slowdowns later in the game.
I have a suspicion that clock-per-clock, the windows tablet and android phone don't perform quite the same (windows should be >faster< due to x86), but lets assume its close. At 800 MHz, you'd expect to be limited to really tiny maps with few vehicles. Roughly 128x256 max, or 128x128 for approximately x2 the speed.
I don't think you can directly compare the original game with OpenTTD in terms of performance cause the original game was written in assembly for efficiency, and a lot fewer features.
My main Android machine is Nvidia Shield Tablet, with Tegra K1 2.2GHZ quad core 2GB memory, and I also play ottd on Xperia Arc, CyanogenMod 13, 1.4GHZ 0.5GB memory.
The Shield tablet works great with ottd, but it is heavy when downloading something. And the Xperia is Extremely heavy, is unplayable with 512^2 map.
I recommend to play ottd with at least 1.5GHZ dual core machine.