Tomato and Chilli plants slow growing… not perceptively grown in weeks!

I sowed these tomatoes and chillies two months ago now. I got really good germination and potted them on into these trays. But they seem to have stalled in their growth for weeks now! They’re usually on a south facing windowsill. Have popped them outside while it’s sunny/warm. Why are they being so slow to grow? Is it because they’re inside? Do they need potting on? Or are they just working on roots and not top growth?