Hmmmm... In maths club, I find that subtraction and addition are normally MUCH slower than multiplication and division. Students can't rely on their working memory to just spurt out the answer and many tire from having to work! I'm coming to the conclusion that many students (regardless of their IQ) don't appreciate the real value in learning number partners - to 10, 20, 50 and 100. Once you really know these number partners and use them to scaffold your learning, seemingly complicated algorithms become much easier. Try drilling number partners in the car (you say 6; s/he says 4, you say 17; s/he says 3, etc.) And then also introduce strategies such as 25+9 = 25+10-1... This can also be extended to 25+8 = 25+10-2... I think it comes back to being really really really familiar with the number system and using 10s, 20s, 50s and 100s as major building blocks.
Anyway, just some thoughts... jojo