Getting too slow to repaint sliders

What is the best way to repaint a component which has several sliders?

My VST plugin UI got really slow when I added more than 10 sliders. In the plugin, I used a Timer to call reapint() in 30 Hz. I took a profile using "Time Profiler" on OS X and I found that juce::GlyphArrangement::addLineOfText() consumes around 10% time of the UI thread. That function is called in Slider class to draw a text label. Such a case is bearable for stand-alone GUI apps, but not for VST plugins because it prevent the whole DAW GUI (like a play cursor, level meter, etc..).