Is there any way that the values here be converted to another variable and not string?
sprintf() is a nicer way to do that (imo), however, there's nothing wrong with what you've done-- if you don't have a ton more code, I'd continue to use it. 

It's simple and that's a lot more important that speed most of the time. Also, the likely memory usage isn't enough to worry about. 

In my experience, once I got past 1500 bytes SRAM (dynamic) memory usage, I ran into trouble. But my projects have a ton of code. I've read through the String class code and it's totally reasonable-- you're not likely to experience lag there. 

My bet is that the lag is due to the timeout period, try making it smaller.