BS String gadgets are very slow when it comes to marking/scrolling text using the mouse.
Compare this with ReAction or even Newstring.mcc.
Think speed should increase with the distance the mouse has from the gadget's edge, just like it does with other string gadgets of AmigaOS.
To try, just paste a very large string in the gadget. Jump to the end. Click LMB and move mouse to the other end. You will notice the slowness.