The next issue here is that EEPROM memory only supports a limited number of writes. Sometimes, the maximum write count is for each individual byte. Sometimes for a block of four bytes. Sometimes for larger blocks.

If your program is constantly too slow, I get the feeling that you are constantly copying data to the EEPROM.

You should only save variables that has been changed. And if the variable changes often, then you should consider how often you really need to copy the contents into EEPROM. Maybe it is enough to save the variable every 10 seconds? But the important thing here is that most variables in a program tends to be quite "temporary" - they may keep their value for very long times, but the value isn't needed to be kept past a reboot since it can be recreated from other data.

If you really do need very large number of writes, then you should not use EEPROM but look at F-RAM memory. They exist either as 8-bit memory looking like normal RAM, or with I2C or SPI serial interfaces. They don't have a limited write count, and they don't consume any extra time for any erase or write cycle making them have similar read and write speeds as normal RAM. With a SPI-connected F-RAM, you can write significant amounts of data to the memory each second and still have time left for other tasks. Especially if the processor has hw support for SPI. And since you have an external display, you probably already have available all the signals for using an 8-bit F-RAM that may be accessed as XDATA.

The thing with F-RAM connected as XDATA is that you don't need two sets of variables - your program can make use of the F-RAM memory directly. Only interrupts or other very high-speed code sections may need the internal data region for the faster OP-codes available.