Finally got around to making a proper optimization.
The program is still not by any means fast (there is still about a half-second delay between input and output on my Ti-84 plus), but it's likely faster than the original.

ClrHome
AxesOff
0->Xmin
Xmin->Ymin
1->[triangle]X
[triangle]X->[triangle]Y
" ->Str1
DelVar KRepeat K=45 or K=105
getKey->K
1+6iPart(.1Ans)+10fPart(.1Ans
Str1+sub("[20 spaces]X[10 spaces]^-1sin(cos(tan(^ ² ()/ log(789* ln(456-  123+  0.~ ",Ans,1->Str1
length(Str1
sub(Str1,1,Ans-(sub(Str1,Ans,1)=" ->Str1
sub(Ans,1,length(Ans)-(K=23->Str1
[line to clear any text currently written]
[line to write Str1 in location just cleared]
End
sub(Str1,2,length(Str1,2,length(Str1)-1->Str1
If K[not equal]45:Pause expr(Str1

Okay, let me tell you this.
This is one serious Rube Goldberg machine of a program. It doesn't need to be on the graphscreen (that just makes it harder), it doesn't really need to use getkey (input would work just fine) and you're just making it really hard on yourself, whether that was your intention or not.

If it needs to be on the graphscreen and use getkey, there's not much I know how to do to help, except for minimally increasing efficiency.
Otherwise, here's a simple program that does the same thing.

ClrHome
clear the graphscreen
Input("",Str1
prompt the user for an input without any question text
expr(Str1
store the expression of Str1 into Ans so it displays at the end of the program instead of "Done"

And there you have your entire program. If you need it to be on the graphscreen and using getkey, sorry about that. But here's a 3-line equivalent to which you can add if you'd like.

Hope this helps.

Thank you very much for the help, but after a Reset, the problem seems to have solved itself. I apologize for not trying that earlier. Despite the occasional bug, the game now works fine!

So, I've recently een hoping to get into graphing calculator game design, and I figured I'd start out by checking out the classics- namely, the original Illusiat.
I downloaded the game generally without incident, storing the two files of the english version in my RAM, and clearing out 10 KB of space. However, after entering my name, I am presented with a domain error. How can I fix this?

Thanks!