Working with Grails and GGTS demands a high amount of memory provided to GGTS. The reason why your IDE is getting slower by the time is because it has to do many "stop-the-world" Garbage Collections to get the memory it needs.
Especially you can see that on opening a GSP file the first time on your IDE ...then GGTS goes dark for a while.
And you can see that on the heap status at the right bottom of your IDE (enable it at your preferences). Every time the used memory is near to your provided 1GB your GGTS is stop working for two or more seconds. After that there is some free memory again.
1GB of memory for GGTS is ridiculous low - 2GB should be the minimum with Grails and Eclipse.
But that's not all. GGTS has a memory leak with Grails Tests and automatically building. You can try to remove the Unit and Integration tests from your projects build path. That is my trick to avoid slowdowns. But if I had to write Unit Tests I have to restart GGTS every once in a while. (And I have 8192m written in my ggts.ini.)