Hi Paspallum,
The older DrumIt modules have an internal SD card slot and can be modified to get an external SD card slot like the DrumIt Five MKII. The modification is quite cheap and easy to install by following guides on this forum (if you're located in Europe contact ANGR77).
Having an external SD card slot allows to upgrade the SD card if needed so if you find that the module doesn't keep up when you play, with a delay between the pad hits and the sound it produces, there is always the possibility to try another card. If wouldn't affect the loading speed of the kits in a noticeable way though. Even if the card isn't the best possible match for the DrumIt the kits are still loaded almost instantaneously when you select a different kit on the module.
If you notice latency when playing bigger kits there are some recommended settings to improve it. Karbonfaiba and halftime found that it could be improved a lot by increasing the Mask setting on the module (increase it as high as possible without missing hits). Setting the Decay to 10 instead of infinite can also help.
The older modules can be upgraded from the 4 GB SD card to 32 or even 64 GB. The size of the kits can vary a lot between the amount of samples and amount of kit pieces but for SD3 and BFD3 it should be possible to store about 30 kits on a 32 GB card. You can use the pieces from different exported kits to create more kit selections on the module or with the 2box Editor or DKit Manager.
I hope that helps, don't hesitate to send me a mail if you have more questions. There's also a small SDSE and DrumIt FAQ here:
https://www.lustark.com/support