The reason it slowed down the tablet, was because the stock RCA ROM was designed and built primarily for that particular model and make of tablet, but a custom ROM may work on the tablet, but it wasn't designed and built specifically for that make and model of tablet and therefore, when the boot sequence starts the OS, tried and fails to enable certain features that maybe present in other devices but are not present in the tablet, and thus waits for a timeout, causing slow sluggish operation, whereas, the stock ROM, only enables modules that are present on the device, also, when someone flashes a custom ROM, there is no stock ROM available anywhere to revert back to, the manufacturers do not support users that flash their devices, and therefore do not have their stock ROMs available to re-flash, plus unlike a PC, there is no way of literally backing up a devices entire stock ROM, so that it can be restored later if anything goes wrong, and in that case, the only solution is the sledge hammer.
That's why I advise people that want to replace their stock ROM with a custom ROM which is already rooted, instead of actually rooting their device, NOT to flash a custom ROM, because as I said, there is no way of backing up the entire stock ROM, also if something does go wrong with the custom ROM and the device is literally 'bricked' and the custom loader doesn't work either, then it's back to the sledge hammer. And that's why I'm never going to flash any of my devices I have now and may get in the future with a custom ROM, unless that ROM was designed and built specifically for a particular device, such as , if there was a way to download a device's stock ROM, then a way to create such a custom ROM would be for that stock ROM to be 'ripped' from the device, modified, then re-flashed, that way the custom ROM would only be for that device.
The problem with custom ROM's is that the ROM developers try to make them as generic as possible, but there is no such thing as 'generic' when it comes to Android devices, as differing manufacturers build their devices differently and as such their stock ROM's are also different too.