Can I add more RAM to the stm32f103c8t6?
Yes you can add more ram, but it won't be quicker as you will only be able to add ram with a serial interface which is slow (because the data and address bytes are sent serially). The STM32f1's don't have parallel hardware interface (FMSC), I am not sure the F3's do but some of the F4's do and F7's and H7's series.  It's better to find a dev board with an fmsc ram built in because wiring up an external ram (40+ wires, and keeping capacitance low) is probably pushing the limits of the most experienced prototyper. 

You might be able to find an F4 in the same package size with more ram since many of ST's product lines have pin compatibility and replace the 103 on your dev board.