Thicker oil making car slower to warm up
 

I’ve just recently had all my seals replaced that were leaking and mechanic has put in a 15W-40 semi synthetic oil saying for an older higher k engine it’s better. But since then I’ve noticed performance on warm up / take off is severely impacted to what it was before? (It had 5W-30). The engine warms up about the same if not actually a little quicker but the performance until the engine is hot is no where good as before.
Before I go changing the oil I just wanted to see if this is a possible cause? I’ve heard thicker oil takes longer for the car to heat up?