OP, really running a 7100t?
The old 64MB BBOS devices can't store many apps to begin with, and it became very easy at the time to fill up the memory and then have low memory issues. But, as already stated, even if you limit the apps to the two or three that you need, the device is still going to be slow. I think the 7100t was GPRS. The 8700 series and similar of that error would be EDGE.
My advice is either update to a modern smartphone or else learn to deal with and tolerate the limitations of a device that is several generations old technology.
And by the way, there is a post on these forums by John Clark that explains how to remove themes and do other removals to free up memory. It isn't much, but when all you have is 64MB in all, removing 2-3 MB is a big deal.
EDIT: Here is what I am remembering... Losing Call Logs, SMS or Emails? Optimize your BB! How To Setup for max free memory!
If you really are using an old 64MB BBOS device, that may help. If you just remove unneeded themes you can free up a MB or so, maybe 2MB.